Solid state ionics: defect interactions and their influence on ionic and electronic transport

Resume : Fuel cells based on ceramic proton conductors receive growing interest because these electrolytes offer a higher ionic conductivity compared to oxide ion conductors, in particular at 300-600 °C. To extend the oxygen reduction reaction to water beyond the three-phase boundary, cathode materials for such cells should have a certain proton conductivity. Since the perovskites considered as cathode material exhibit perceptible oxygen vacancy, proton and hole concentrations, proton uptake can occur by water incorporation (acid-base reaction) and by hydrogen incorporation (redox reaction) [1]. These regimes are explored by thermogravimetry (pH2O changes in different pO2) for perovskites such as La0.5Sr0.5FeO3-d, Ba0.5Sr0.2Fe0.8Zn0.2O3-d. The obtained maximum proton concentrations are significantly lower than in BaZrO3 electrolyte materials at same T and pH2O [2]. The variation of proton concentration with cation composition in (Ba,Sr,La)(Fe,Co,Zn,Zr)O3-d perovskites is discussed. The proton mobility in Ba0.5Sr0.2Fe0.8Zn0.2O3-d extracted from the transient behavior is comparable to that in BaZrO3 electrolytes. [2] D.Poetzsch, R.Merkle, J.Maier, Adv.Funct.Mater. 25 (2015) 1542 [2] D Poetzsch, R.Merkle, J.Maier, Farad.Disc. 182 (2015) 129

Resume : Amongst ceramic fuel cells, protonic ceramic fuel cells (PCFC) attract growing interest. Proton-conducting ceramic electrolytes offer a higher ionic conductivity compared to standard zirconia or ceria oxide ion conductors, in particular at intermediate temperatures of 300-600°C. The two main limiting factors are (i) the manufacturing of a dense, thin electrolyte membrane with non-blocking grain boundaries, and (ii) finding optimum cathode materials for application on a proton-conducting electrolyte which also shows some proton conductivity. So far, proton concentrations were reported only for few cathode materials such as Ba0.5Sr0.5Co0.8Fe0.2O3- (BSCF) and Ba0.5Sr0.5Fe0.8Zn0.2O3- (BSFZ [1]). These proton concentrations were found to be about one order of magnitude smaller than for electrolyte materials such as BaZr1-xYxO3-x/2 under comparable conditions. In the present study, we performed first principles calculations (CRYSTAL09 computer code with hybrid PBE0 functional [2]) for nonstoichiometric La1-xSrxFeO3- as a potential PCFC cathode material, with and without protons. These calculations yield detailed information on various possible point defect configurations. This makes it possible to look for correlations of their formation energies with ionic charges, local atomic coordination, bond lengths etc., in order to obtain an atomistic understanding of the key parameters determining proton uptake in PCFC cathode materials. Proton migration and the comparison to proton uptake in other cathode materials will also be discussed. [1] D. Poetzsch, R. Merkle, J. Maier, Faraday Discussions 182 (2015) 129 [2] R. Dovesi, B. Civalleri, R. Orlando, et al., Rev. Comput. Chem. 21 (2005) 1.

Resume : During the last decades, perovskite-type anhydrous oxides have received large attention as potential electrolytes and electrodes able to operate at intermediate temperatures for Proton Ceramic Fuel Cells (PCFC), gas separation membranes and Steam Electrolysers (SE) [1]. The use of medium gas pressure allows optimizing the economic efficiency of the above mentioned devices. Materials life-time is a key parameter. Previous studies, performed in collaboration with industrial partners [2], showed however that high water pressure can significantly accelerate a material ageing. Consequently, our protonation procedure performed in autoclaves under high water pressure [3,4] can be considered as an accelerated stability test which allows very fast to choose the most stable. The structural and chemical stability of well-densified ceramics (zirconates-cerates, nickelates and cobaltite-ferrites [5-7]) exposed during week(s) to CO2-rich or CO2-free water vapour pressures (20 to 40 bar) at 550°C in an autoclave device have been studied using various techniques: TGA, dilatometry, XRD, Raman scattering and IR spectroscopy. The comparison allows the choice of the most pertinent composition for industrial applications requiring a selected material with important life-time. The results show the presence of traces or second phases originating from undesirable hydroxylation and carbonation, detected in the near-surface layers. The proton/water insertion modifies the structure symmetry and the unit-cell volume whatever its low amount (<0.5 wt% equivalent H2O). [1] Ph. Colomban, Proton Conductors, Cambridge University Press, 1992 (1st ed.), 2008 (paperback), 2011 (e-book). [2] Patents FR2916653 (B1) 05/12/2008, WO 2008152317 (A2) ; EP 2168198 (A2); EP 12773302.0-1360 [3] A. Slodczyk et al., Membranes 3 (2013) 311. [4] A. Slodczyk et al., J. Physics & Chemistry of Solids 83 (2015) 85. [5] A. Slodczyk et al., Solid State Ionics 262 (2014) 870. [6] S. Upasen et al., J. Alloys Compounds 622 (2015) 1074. [7] S. Upasen et al., Ceramics International 41 (2015) 14137.

Resume : Dynamic segregation processes in perovskite electrodes have been studied in operating electrochemical cells at high temperature by spatially resolved surface spectroscopy. The electrochemical cells contained zirconia electrolyte and mixed perovskite catalyst. Surface segregation, such as enrichment of silica and other glass-forming impurities at zirconia electrolyte surfaces and strontium segregation in (Sr, La)MnO3 perovskite electrodes as result of high temperature annealing have already been reported in the past. In this work, we conducted a broad study on the impact of cell polarization under operation conditions and found that many impurity segregation processes are reversible, redistribution of impurities between different surfaces takes place and that electrolyte and electrode surfaces can even be cleaned under extreme applied potentials. The impact of such dynamic changes in surface chemistry on oxygen exchange processes were also investigated.

Resume : The mixed ionic-electronic conducting ceramic La2NiO4 δ (LNO) has been proposed as promising material for various applications such as electrodes for solid oxide fuel cells (SOFC) and electrolyser cells (SOEC) as well as for ceramic membranes. LNO exhibits fast oxygen diffusion, high catalytic activity for the oxygen surface exchange reaction and sufficient electronic conductivity. Since oxygen transport through the material is based on an interstitial diffusion mechanism, acceptor doping with alkaline earth metals is not required to enhance the oxygen ionic conductivity. This might entail better long-term stability with respect to various well-known degradation mechanisms such as surface or grain boundary segregation of alkaline earth elements or reaction of alkaline earths with impurities in the gas phase. In this work LNO is investigated regarding its applicability as electrode in SOFC/SOEC-cells. Results are obtained from electrochemical impedance spectroscopy and current-voltage analyses on symmetrical cells with screen printed LNO electrodes under anodic and cathodic polarisation including chromium poisoning effects. A study of the long-term behaviour of the electrodes in pure Ar/O2 as well as in humidified atmospheres is performed at 800°C. A pronounced change in the electrode performance is observed under cathodic polarisation and is correlated with results from post-test analysis by scanning and transmission electron microscopy.

Resume : LaMnO3-based perovskites have been widely studied as the oxygen electrode for high temperature solid oxide fuel cells (SOFCs). Sr-doped LaMnO3 is the conventional cathode material for high temperature operation due to its high electrical conductivity, good activity for the oxygen reduction reaction and compatibility with electrolyte materials.[1] Due to the high costs and accelerated performance degradation associated with SOFCs operating at high temperatures, further research is required to reduce the operating temperature and improve their efficiency. The ionic and electronic conductivity of LaMnO3 at intermediate temperature (600 – 1000 K) can be improved by introducing lower valence dopants at both the La and Mn sites.[2] Here, we investigate doping with alkaline earth metals, Mg, Ca, Sr and Ba. The formation energies of oxygen vacancies and alkaline earth defects in LaMnO3 have been calculated using PBEsol + U.[3] Isolated defects and clustered pairs have been investigated at both La and Mn sites, to establish the most probable site at which they will be introduced. The charge compensation mechanism has been examined by considering both ionic (oxygen vacancy formation) and electronic compensation (hole localised at the Mn site). [1] J. A. Kilner and M. Burriel, Annu. Rev. Mater. Res., 2014, 44, 365 – 393 [2] L. Gan et al., J. Alloys Compd., 2016, 655, 99 – 105 [3] J. P. Perdew et al., Phys. Rev. Lett., 2008, 100, 136406

Resume : SOFC is considered as one of the most promising technologies in the production of clean energy. In the search for new MIEC materials for efficient electrodes, the Ruddlesden Popper manganites n=1 are of high interest, as those compounds are able to accommodate a significant amount of non-stoichiometric oxygen, besides presenting high thermochemical stability in reducing conditions (800°C, pO2 = 10-10 Pa). In this study, Sr2-xLaxMnO4±δ (0.25, 0.4, 0.5 and 0.6) powders were prepared by traditional solid state reaction in air. The samples were characterized by XRD and the structural informations obtained from Rietveld refinement are in agreement with literature. The Mn oxidation states after synthesis as well as the oxygen loss due to Mn reduction were determined by iodometric titration and TGA analysis in hydrogen, respectively. The position of the oxygen defects within the crystal structure have been examined using Neutron Powder Diffraction technique as a function of temperature in air and hydrogen. The total electrical conductivity was measured by four-probe DC technique, in air and 2% H2 from 800°C to room temperature, showing a p-type behavior with activation energy values characteristic of a small polaron mechanism. Acknowledgments: This work was developed with the support of COLCIENCIAS (project 110265842833 - contract 038-2015), LLB, CNRS, Région Nord Pas-de-Calais and MENESR, Laboratorio de Rayos X (UIS).

Resume : Co-electrolysis of steam and CO2 is now attracting much interest for recycling of carbon and high temperature co-electrolysis has an advantage of small electricity input for production of synthesis gas (CO/H2). At present, Ni based catalyst is widely used for cathode of electrolyzer, however, because of coke deposition and low activity to CO2 electrolysis, cathode catalyst with high activity is now strongly demanded. In this study, we investigated LaFeO3 doped with Sr and Mn as an active catalyst for CO2 electrolysis and it was found that Sr and Mn doped LaFeO2 shows electron and oxide ion conductivity under electrolysis condition and high activity to CO2 electrolysis. We found that a LaFeO3−δ-based perovskite cathode showed the high activity to H2O and CO2 electrolysis among the examined oxides with a current density for CO2 electrolysis of 0.52 A/cm2 at 1.6 V and 1173 K. Doping of LaFeO3 with Sr and Mn was highly effective for increasing the cathode activity for CO2 electrolysis under the feed gas condition of 50% CO2, 1%CO, 49% Ar. The porous structure with fine particles of the LSFM6482 cathode was sustained under CO2 electrolysis conditions with little phase separation and no carbon formation. Application of LSFM for H2O/CO2 co-electrolysis was further studied and it was found that CO rich gas can be obtained under co-electrolysis condition. On Ni based catalyst, the products are always enriched with H2 by water gas shift reaction (H2+CO2=CO+H2O) but in case of LaFeO3 oxide cathode, CO rich gas was mainly formed suggesting low activity to gas shift reaction and easily control the CO/H2 ratio in product. Active site in LaFeO3 will be discussed from defect chemistry.

Resume : Defect interactions play a crucial role in determining many properties of ceramics and yet often remain poorly understood. Experimentally they are difficult to study; computational work has largely been restricted to particular clusters chosen in advance and the dilute (point-defect) limit in which defect clusters are embedded in the undefective lattice. The number of configurations and the size of the defect clusters have been limited and temperature effects are often neglected. In this presentation we review new hybrid Monte Carlo (HMC) simulations which suffer none of these restrictions. These combine Monte Carlo and molecular dynamics techniques in order to explore phase space efficiently and simulation cells containing many thousand atoms are feasible. As an example, results are presented for Gd-doped CeO2 as a function of Gd concentration and examine the growth of Gd-rich domains. Even at low concentrations the Gd3+ ions segregate into domains. The local environments of the Gd3+ ions are consistent with the formation of cubic C-type (Gd2O3) domains, rather than the monoclinic B-type or pyrochlore clusters suggested previously. Earlier detailed pair distribution function analysis of the solid solution indicated different local cation environments from those suggested by a Rietveld analysis – overall our results support the former rather than the latter. At the elevated temperatures (1000 K) of the simulations there is no particular preference for vacancy and dopant cations to be located at second neighbour sites, an issue long debated for this and similar systems. Both calculated and experimental conductivities show a similar variation with composition, passing through a maximum with increasing Gd concentration. The conductivities of the configurations generated in the HMC are lower than those of configurations, generated independently, in which the Gd ions are distributed at random. The HMC thermally generated Gd nano-domains capture oxygen vacancies, reduce oxygen vacancy mobility and block diffusion paths. We also outline recent results using a new adaptive Kinetic Monte Carlo simulation code (DL_aKMC) for doped cerias and zirconia. aKMC provides a distinct advantage over traditional KMC methods, in which one has to provide a list of system state transitions in advance. Here all of the state transitions are dynamically generated, leading to a more accurate simulation of the kinetics as the system evolves.

Resume : Current solid oxide fuel cells (SOFCs) require temperatures in the region of 1000°C to operate. This is primarily due to the fact that current generation SOFC electrolytes, such as ytrria stabilized zirconia (YSZ), require high temperatures in order for sufficient ionic diffusion of the oxide ions to occur. It has been suggested that replacing YSZ with samarium doped ceria (SDC) would reduce the operating temperature of SOFCs into the intermediate temperature range (600-800°C), thus greatly reducing operating costs and decreasing device degredation.[1] Molecular dynamics can be used to investigate ionic conductivity and its limitations in these systems. Here, we compare the performance of two interatomic potentials derived for SDC from ab initio data, a dipole polarizable ion model (DIPPIM) and a rigid ion model (RIM).[2] The DIPPIM allows for polarization effects resulting from induced dipoles whereas the RIM does not. In this study we aim to elucidate whether or not this system can be modelled successfully using a RIM or if a DIPPIM is necessary due to the presence of oxide ions. There has been little theoretical investigation into oxide ion conductivity of SDC at interfaces, which is essential to the performance of this material as a SOFC electrolyte. Here we discuss the effect of low-index surfaces on the performance of SDC as an oxide ion conductor. [1] Inaba et al, Solid State Ionics, 83, 1-16 (1996) [2] Castiglione et al, J. Phys: Condens. Matter, 11, 9009-9024 (1999)

Resume : Ceria (CeO2) has a vast variety of technological applications, ranging from energy storage solutions to catalysis and memristive devices. The huge interest in the material is caused primarily by the good oxygen-transport properties of acceptor-doped ceria, with the mobile charge carriers being oxygen vacancies. In this study, oxygen-ion transport in ceria is investigated with classical molecular dynamics calculations employing robust empirical pair potentials. The transport of oxygen vacancies is compared with that of oxygen interstitials. Activation energies of migration are determined both from static nudged-elastic-band calculations and from the temperature dependence of the diffusion coefficients obtained from molecular dynamic simulations. In order to obtain a more comprehensive understanding of the oxygen transport processes, the drift of oxygen ions in the presence of an external electrical field is also studied. The resulting conductivities and drift velocities are compared with analytical predictions and experimental data. Furthermore, a comparison of the conductivities with the diffusivities allows for a discussion of potential correlation effects associated with the oxygen transport in ceria.

Resume : There is ever growing interest in the modification of the transport properties of ionic conductors when grown in confined systems such as thin films. It has now been over a decade since the first tantalising results were published, initiating widespread interest in the potential of oxygen ion conductors at the nanoscale. Much of the work has focused on commercial electrolytes such as yttria-stabilised zirconia and gadolinia- or samaria-doped CeO2, which have been extensively studied as bulk materials. These electrolytes are so-called ‘optimised’ ionic conductors, as the size of the dopant cation is such that the vacancy-dopant interactions are minimised, resulting in high conductivities. Although it is thought that the interplay of lattice strain and defect association should play a role in the transport properties of the doped fluorites, this has not been explicitly studied. We have fabricated doped-CeO2 films by pulsed laser deposition on sapphire substrates. The dopants used were La, Gd, and Yb, which represent an ionic radii mismatch with the host Ce lattice of 19.6%, 8.6%, and 1.5% respectively. Employing out-of-plane and in-plane x-ray diffraction, raman spectroscopy, and transmission electron microscopy we show that the lattice strain in the films can be controlled by varying the film thickness. From the transport properties, as characterised in-plane by impedance spectroscopy, the effect of lattice strain on the defect association in doped-CeO2 will be discussed.

Resume : Doped ceria is of particular scientific interest due to the large variety of possible applications in chemical catalysis, automotive exhaust purification, and energy conversion devices such as solid oxide fuel cells, electrolyser cells and rechargeable oxygen batteries. The most prominent properties of doped ceria are its high oxygen ion conductivity and high reducibility. The reduction of ceria leads to the formation of oxygen vacancies and small polarons and is influenced by the mutual interaction of defects. In this study we investigate the interaction and distribution of defects in doped non-stoichiometric ceria Ce1 xRExO2 x/2 δ for different rare-earth dopants by means of DFT+U calculations and Monte Carlo simulations. We describe this system by the pair interaction energies of the prevalent defect, i.e. dopant ions, oxygen vacancies, and small polarons. The simulations reveal that in thermodynamic equilibrium the configurational energy of the lattice decreases with increasing non-stoichiometry as well as increasing dopant fraction due to the interaction of oxygen vacancies with dopant ions and small polarons. This effect is similar to the observed decrease of the enthalpy of reduction in experiments. Consequently, the interaction of defects in doped ceria does not only influence the ionic conductivity of the material but also its reduction behaviour.

Resume : Materials with three mobile carriers (oxygen vacancies, protons, holes) are of interest for proton-conducting ceramic fuel cells. The incorporation of protons into such mixed-conducting oxides can occur by acid-base hydration as well as by hydrogen uptake.[1] Depending on conditions, one-fold or two-fold conductivity relaxation after pH2O steps is observed.[2-4] For a complete description four diffusion coefficients are required, comprising direct as well as "indirect" terms. The complex non-monotonic kinetic behavior is related to the fact that in a three carrier system the electroneutrality condition does not lead to a simple coupling between the carrier fluxes. Numerical simulations for a wide range of materials are presented.[1,5] They allow us to identify the conditions for the transition from one-fold to two-fold relaxation, and give a natural explanation for the "moving boundary phenomenon" observed in ref. [2]. [1] D.Poetzsch, R.Merkle, J.Maier, Adv.Funct.Mater. 25 (2015) 1542 [2] J.H.Yu, J.S.Lee, J.Maier, Angew.Chem.Int.Ed. 46 (2007) 8992, Solid State Ionics 181 (2010) 154 [3] H.I.Yoo, J.Y.Yoon, J.S.Ha, C.E.Lee, Phys.Chem.Chem.Phys. 10 (2008) 974 [4] E.Kim, H.I.Yoo, Solid State Ionics 252 (2013) 132 [5] R.Merkle, R.Zohourian, J.Maier, Solid State Ionics (2015) accepted

Resume : Stable to carbonization praseodymium nickelates-cobaltites (PNC) and their nanocomposites with Y-doped ceria (YDC) are promising materials for intermediate temperature solid oxide fuel cells (IT SOFC) cathodes and oxygen separation membranes. However, conventional sintering (CS) technique does not allow to obtain their mechanically strong/dense layers at temperatures within the range of PNC phase stability. In this work microwave (MWS) and radiation thermal (RTS) sintering techniques were applied for these systems. Oxide powders were obtained by Pechini route. Nanocomposites were prepared via ultrasonic dispersion of powders in isopropanol. All materials were sintered in air up to 1300°C and characterized by XRD and TEM. The oxygen mobility was studied by oxygen isotope heteroexchange with C18O2. For PNC samples sintered at T<1100°C by CS and RTS the main phase was orthorhombic perovskite. For MWS samples the phase composition varies depending on the heating rate and sintering time. The lowest residual porosity for MWS PNC–YDC samples was 4%. For nanocomposites sintered by all methods strong Pr cation transfer into YDC provides the main channel of fast oxygen diffusion (Do up to 10-7 cm2/s at 700°C) via Pr-Y-Ce-O domains and their interface with PNC. Hence, advanced sintering techniques providing a higher density of nanocomposites without impairing their functional characteristics are promising for design of IT SOFC and oxygen separation membranes.

Resume : La0.6Sr0.4FeO3-d (LSF) is a very attractive electrode material for solid oxide fuel cells (SOFCs), due to its high ionic and electronic conductivity and the catalytic activity for oxygen exchange. In this study, electrochemical impedance spectroscopy (EIS) was used to investigate the electrochemical properties of LSF thin film model electrodes under polarization and in different atmospheres to gain information on the oxygen exchange kinetics and defect chemistry. Well defined thin film electrodes of 200 nm thickness were prepared on yttria stabilized zirconia single crystals (100) by pulsed laser deposition (PLD). EIS measurements, as well as the combination of oxygen partial pressure and overpotential variation were used to investigate the dependency of defect chemistry on polarization and oxygen partial pressure. Charge carrier concentrations were found to solely depend on the electrochemical potential of oxygen in the electrode, whether it is driven chemically (oxygen partial pressure) or electrically (polarization). Three point DC measurements on these electrodes showed different current voltage characteristics for oxygen incorporation and release (i.e. anodic and cathodic polarization). While the oxygen incorporation exhibits strong dependency on the oxygen partial pressure, the oxygen release was found to be mostly governed by the electrochemical potential of oxygen in the electrode.

Resume : Lithium oxide excess and powder bed method are conventionally used for synthesis of lithium-conducting solid electrolytes with cubic garnet structure. In this work Li6.75La3Zr1.75Nb0.25O12 garnet was sintered at 1200°C on alumina and zirconia supports with controlled amount of lithium oxide excess. It was found that the powder bed method did not prevent garnet pellets from alumina contamination. The quantity of alumina penetrated into samples correlated with the amount of lithium oxide excess added and varied from 0.4 to 0.9 wt. %, as indicated by ICP AES. In contrast to alumina, zirconia did not affect chemical composition of the pellets. Taking this into account a modified powder bed method that permits to eliminate any alumina contamination was developed. However, non-contaminated samples showed lower density after sintering when compared to contaminated ones. Lithium oxide excess in the samples is likely to react with the alumina with formation of eutectic that melts at 1200°C. The liquid eutectic phase acts as a sintering aid and promotes densification up to relative densities of 92%. Electrical properties of the pellets were measured by EIS technique. Bulk conductivity of the dense pellets was 5×10-4 S/cm at 25°C with the activation energy of 0.3 eV and depends on density. Due to grain boundary resistance dc-conductivity of pellets was lower than 10-8 S/cm at 25°C with the activation energy of 0.8 eV. Possible reasons of high grain boundary resistance are discussed.

Resume : Materials for new energy applications have attracted a number of recent research interests because of the concern about depletion of fossil fuels and construction of sustainable societies. In particular, all-solid-state lithium ion rechargeable batteries (LIBs) with higher energy density are indispensable for next-generation energy storage devices such as electric vehicles, hybrid electric vehicles, and smart grids. A large part of components of these devices is generally inorganic matters possessing crystalline form. Since crystal shape, size and crystallinity drastically affect various LIB properties, controlling them at the same time is required. In this regards, our group have researched “nature-mimetic flux growth” of inorganic crystals for these applications. Flux growth is a kind of liquid phase crystal growth technique, in which molten metals and molten metal salts are used as solvents. As the growth conditions of inorganic crystals are similar to those in earth’s crust, we call it “nature-mimetic”. Characteristic features of flux grown crystals are high-quality without thermal strain, idiomorphic shape with specific crystal plains, and controlled shape and size. Highly crystalline layers of electrodes and solid electrolytes for LIBs such as LiCoO2, LiNi0.5Mn1.5O4 Li4Ti5O12, Li7La3Zr2O12, and Li5La3Nb2O12 have been successfully grown by flux coating method. It has been found that flux coating method is effective for high-quality crystal layers on various substrates.

Resume : The change from solid oxide fuel cell (SOFC) cathodes with surface path kinetics to mixed conducting cathodes exhibiting a bulk path was one of the most important achievements within the last decade of SOFC research. Also in case of anodes the transition to mixed conducting systems is expected to further increase SOFC’s performance. A promising material for this application is (La,Sr)(Cr,Mn)O3 (LSCrM), which provides the nice opportunity of two possible ways of changing its electronic properties: (i) the Sr-doping on the A-site and (ii) the Cr:Mn ratio on the B-site. In this study LSCrM thin films with both varying Sr content and Cr:Mn ratio were prepared by pulsed laser deposition. Conductivity measurements were carried out in O2 as well as H2/H2O atmospheres using the Van-der-Pauw-method. The obtained conductivity data are discussed in terms of the material’s defect chemistry. Low Mn contents, for example, decrease the conductivity drastically, most likely due to trapping of charge carriers on the Mn sites. With higher Mn contents, percolation occurs and much higher conductivities can be obtained. In the second part of the study selected compositions were also investigated as model-type electrodes on zirconia electrolytes in H2/H2O atmosphere. Electrochemical behavior was characterized by means of impedance measurements and elementary properties such as surface exchange resistance and chemical capacitance are discussed referring to the material’s conductivity.

Resume : In oxygen transport membranes (OTM), the dense ceramic separation layer is a fundamental component and is intended to produce oxygen with high selectivity. To function with high efficiency, both high electronic and ionic conductivity are required. For single phase Mixed Ionic-Electronic Conducting (MIEC) materials, because of the inevitable restrictions e.g. stability and durability, the dense layers are yet to achieve the performance required for an OTM. Therefore, accumulated attention has been focused on dual phase composites in order to fulfill the requirements. In the current work the dense layer consisting of Lanthanum Strontium Chromium Ferrite (LSCrF) based perovskite as the electronic pathway and Scandia Stabilized Zirconia (ScSZ) as an ionic conductor has been investigated. Among all the essential criteria, transport properties are profoundly important aspects. In practice, the electrical conductivity and ionic transport property were measured. The total electrical conductivity was measured by the 4-point DC method up to 1000°C in a static air atmosphere. The experimental technique used to determine the oxygen transport kinetics was Isotopic Exchange Depth Profiling (IEDP) performed using Secondary Ion Mass Spectrometry (SIMS)[1]. Whilst the OTM can function effectively with these materials, the detailed mechanism by which the dense layer transports oxygen is unclear. Thus, in current work, the fundamental transport properties of the dense composite layers have been investigated. Moreover, in order to achieve the optimal properties, variations on different phase ratios and microstructures have been performed and effects of different environment will also be presented. References [1] J. A. Kilner et al., Journal of Solid State Electrochemistry, 15 (2011) 861-876.

Resume : Because of the high chemical stability of strontium tungstate and the good catalytic properties of cerium compounds, mixed strontium-cerium tungstates, SrxCey(WO4)z, are good candidates for the methane transformation into syngas (H2+CO) in a membrane reactor. This type of reactor involves complex mechanisms of ionic, electronic conduction and catalytic oxidation based on the charge carriers transfer. The properties of binary tungstate SrWO4 and Ce2(WO4)3 were compared to those of the new ternary compound SrxCey(WO4)z. Powders have been synthesized using a modified EDTA-citrate methodology and were characterized by X-Ray diffraction, thermal analysis, electron microscopy techniques, and specific surface area analyses. The different tungstates exhibit good crystallization, and are single phased. The electrical conduction has been determined from electrical impedance spectroscopy analyses in the temperature range 300°C-950°C, using AC and DC mode polarization. The different tungstates powders exhibit mixed electronic and ionic conduction, with different activation energies. Measurements were also performed under partial pressure of oxygen, in order to evidence the oxygen role in the methane oxidation. The catalytic activity was determined as a function of time, temperature and methane concentration, under oxidizing (air) and reducing (argon) atmospheres. Binary as well as ternary compounds have a good catalytic activity for the partial methane conversion. The catalytic activity of the samples was correlated to the enhanced mixed conduction observed above 450 °C. Acknowledgments: This work was done in the general framework of the CAPES COFECUB Ph-C 777-13 french – brazilian cooperation project.

Resume : The state-of-the-art perovskite solid solution Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) possesses formerly unobtainable oxygen transport properties in its cubic phase. This has made BSCF a material of choice for high-temperature oxygen-transport membranes (OTMs). However, cubic BSCF exhibits a limited long-term phase stability at T ≤ 840 °C, forming secondary phases that result in a degradation of its oxygen permeability. This study focuses on a systematic analysis of the influence of B-site Y doping on the behavior of the BSCF system. Stability at low pO2 (at 900 °C), lattice parameters (20…900 °C), long-term conductivity degradation (700…900 °C), oxygen permeation (650…1000 °C), and microstructural changes have been determined depending on Y content. To this end, single-phase BSCF powders doped with 1…10 at% Y on the B-site were prepared by mixed-oxide route. Dense ceramic pellets were sintered and used for electrical conductivity and oxygen permeation measurements as well as for long-term annealing procedures at various temperatures for subsequent SEM/TEM analyses. The conductivity degradation for pure BSCF at 800 °C (~ 3 % after 800 h) could be completely suppressed by addition of ≥ 3 % B-site Y. SEM and TEM studies demonstrate the complete suppression of the hexagonal phase in the case of high dopant concentrations (10 at%) at a temperature of 800 °C. The oxidation state of the multivalent cations as well as A/B-site occupation for Y could also be determined to gain further insights into the causes for the stabilization of the cubic phase by Y-doping.

Resume : For an application of mixed ionic-electronic conducting (MIEC) perovskite oxides, e.g., as solid oxide fuel cell (SOFC) cathodes, as high-temperature gas sensors or as oxygen-transport membrane (OTM) materials, the kinetics of oxygen transport is of fundamental importance. A common method for the determination of the chemical diffusion coefficient D_chem and the surface exchange coefficient k_chem is electrical conductivity relaxation (ECR). Usually the conductivity response of a sample is measured after abruptly changing the ambient oxygen partial pressure pO2 using different gas mixtures. In this study, however, a closed tubular zirconia “oxygen pump” setup was used which facilitates precise pO2 control with a high resolution at temperatures above 700 °C in atmospheres ranging from pure oxygen continuously down to pO2 = 10E-18 bar. Reasonably fast pO2 changes enable an application of the ECR technique on MIEC oxides down to lower pO2 not easily accessible with gas mixtures. The oxygen transport parameters of dense ceramic bulk samples of Ba0.5Sr0.5Co0.8Fe0.2O3-d (BSCF), La0.58Sr0.4Co0.2Fe0.8O3-d (LSCF), and Pr0.58Sr0.4Co0.2Fe0.8O3-d (PSCF) have been studied at high temperatures in the range between 10E-6 ≤ pO2 / bar ≤ 0.21. The D_chem and k_chem values obtained for LSCF at 800 °C are in good agreement with values from literature, proving the usability of the setup for ECR measurements. For BSCF, LSCF, and PSCF, D_chem and k_chem values could be determined for the first time at 900 °C as a function of pO2.

Resume : Ba0.5Sr0.5Co0.8Fe0.2O3-d (BSCF) is a mixed oxide with excellent oxygen-transport properties destining it for use in high-temperature applications such as oxygen-transport membranes. Although determination of electrical conductivity as a function of temperature T and oxygen partial pressure pO2 seems a more or less “trivial” task, it has to be kept in mind that conductivity changes stem from thermodynamically changing oxygen stoichiometries. At sufficiently high temperatures (> 600 °C) or under oxidizing conditions (> 10E-4 bar), where oxygen exchange is reasonably fast, equilibrium conductivity values are reached in seconds or minutes, depending on sample dimensions. At intermediate temperatures or in more reducing atmospheres, however, the mechanisms of oxygen exchange slow down significantly, leading to fairly long equilibration times of up to several tens of hours for a dense ceramic bulk sample with a thickness in the order of 1 mm. This means that, e.g., in temperature-sweep measurements, conductivity exhibits a hysteresis behavior and true equilibrium values below 600 °C can only be determined by suitable dwelling times. A similar behavior is observed in conductivity measurements at 900 °C in reducing atmospheres (pO2 = 10E-8…10E-6 bar) where several hundreds of hours are required for equilibration. This behavior is an indicator of the strongly decreasing values for the transport parameters at low pO2 and is not a result of any sample deterioration (phase transition or decomposition).

Resume : Mixed ionic-electronic conducting (MIEC) perovskite oxides like Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) are the usual choice in high temperature applications, such as porous functional coatings in oxygen transport membranes (OTMs) for gas separation and porous electrodes in solid oxide cells (SOCs) due to their excellent oxygen-ionic and electronic transport properties. However, since the chemical oxygen diffusion coefficient (Dδ) and the oxygen surface-exchange coefficient (kδ) are usually determined for dense bulk materials (i.e. electrical conductivity relaxation (ECR) measurements) little is known on the influence of different microstructural parameters and/or operating conditions. This contribution couples both electrochemical impedance spectroscopy (EIS) and Focused Ion Beam (FIB) tomography to determine the oxygen transport properties Dδ and kδ of BSCF symmetrical cells, with different microstructural parameters (porosity and active surface area) and different time or thermal history. The performance of these BSCF electrodes is fully characterized by electrochemical impedance spectroscopy from 600 to 900 °C and conducting experiments with varying the oxygen partial pressure (pO2 = 0.02...1 atm). By analyzing the distribution function of relaxation times (DRTs) the individual processes taking place are identified and a physically meaningful equivalent circuit model can be established. Values of porosity, surface area and tortuosity obtained from the FIB tomography are used to calculate the specific Dδ and kδ. The values will be critically compared with the literature values towards a better understanding and quantification of the processes taking place in porous BSCF layers applied either in OTM or SOC.

Resume : Solid oxide fuel cells (SOFCs) are a strong candidate for zero-emission electricity generation. However, the high temperatures necessary for operation lead to long start up times and short lifetimes. Intermediate temperature SOFCs suffer from a decreased rate in the oxygen reduction reaction (ORR) at the cathode site, leading to an interest in mixed ionic electronic conductors (MIEC) as cathode materials which can increase the area within which the ORR can take place, increasing the reaction rate. In this paper we discuss intrinsic point defects, substitutional defects and oxide ion migration in LaFeO3, a promising MIEC cathode material, studied using molecular mechanical and density functional theory calculations. The majority of intrinsic defects investigated have high formation energies with the key exception being O2- vacancies. A range of divalent substitutional defects have been investigated for both the A- and B-site and the most appropriate for each will be discussed, along with their affect on the electronic properties of the material. Oxide ion conduction calculations revealed small activation energies along the majority of paths between two oxygen vacancies, with values that agree well with experiment.

Resume : Sodium chalcogenide and chalcohalide glasses exhibit high ionic conductivity and can be used in all-solid-state batteries, chemical sensors and other devices. However, the available information on NaCl-containing chalcogenide glass systems is relatively poor. In this work we will present conductivity and structural studies for (NaCl)x(Ga2S3)y(GeS2)100-x-y glasses, where sodium concentration changes by 4 orders of magnitude from 0.001 to 50 mol.% NaCl. Dc conductivity and ac impedance measurements have been used to study the electric properties of this glassy system. The room temperature conductivity increases by 11 orders of magnitude with increasing sodium concentration, ranging between 10^(-17) S/cm (x = 0) and 10^(-6) S/cm (x = 40). The activation energy decreases from 1.00 to 0.47 eV, respectively. The composition dependence of the ionic conductivity σ_i (x) shows two drastically different transport regimes at low (x ≤ 10) and high (x > 10) sodium concentrations. A characteristic feature of the conductivity isotherms in diluted glasses is a power law dependence of the ionic conductivity σ_i (x) ∝x^(To/T), where To is the critical temperature related to connectivity of the host glass. The rapid increase of conductivity in a rather limited concentration range and the power law dependence of σ_i (x) are thus caused by a percolation-controlled mechanism. Deviations from the critical percolation regime are observed above 10 mol.% NaCl. A possible change in the ion transport mechanism seems to be the origin of the observed trend. Raman spectroscopy, neutron and high-energy X-ray diffraction results will be used to relate different transport regimes and glass structure.

Resume : Mixed-conducting La0.6Sr0.4CoO3-δ (LSC) is an attractive cathode material for solid oxide fuel cells (SOFCs) operated at 600-800°C. However, the long-term stability of LSC is limited due to Cr- and Si-poisoning by impurities from the SOFC stack. In the present work the oxygen exchange kinetics of LSC was investigated in-situ for 3500 h at 800°C in dry and humidified atmospheres. The chemical diffusion coefficient (Dchem) and the chemical surface exchange coefficient (kchem) of oxygen were studied in the absence and presence of Cr- and Si-sources. Degraded samples were analyzed by scanning electron microscopy (SEM) with energy and wavelength dispersive X-ray spectroscopy (SEM-EDXS/WDXS), X-ray photoelectron spectroscopy (XPS), and analytical scanning transmission electron microscopy (STEM). In dry atmosphere with 10 % O2 high values of Dchem=2E-5 cm2 s-1 and kchem=1E-3 cm s-1 were found at 800°C. Stable performance was observed during 1300 h without or with Cr- and Si-impurities. However, a significant decrease in Dchem and kchem occurred when the atmosphere was humidified (30-60 % relative humidity). XPS depth profiles showed Sr-enrichment and Co-depletion of the surface (10-20 nm depth) already during 1300 h in dry atmosphere. After the treatment in humidified atmospheres for additional 2000 h significant amounts of Cr were found to depths of 800-900 nm. SEM and STEM showed crystallites of Cr- and Si-rich secondary phases on the surface and at the grain boundaries in the near-surface region. It can be concluded that the observed decrease of the oxygen exchange kinetics is closely related to significant changes of the surface composition of LSC as a result of the decomposition of the bulk phase into inactive secondary phases.

Resume : A cross plane fully integrated micro oxygen sensor based on a self-sustained ion conducting membrane has been developed. The micro sensors were fabricated combining industrial clean room micro fabrication process and the use of nanometric thin films of Yttria Stabilized Zirconia (YSZ). Large area YSZ membranes of 200 nm thickness were deposited by pulsed laser deposition (PLD) at wafer level. The thermomechanical stability of the self-supported ultrathin membranes was studied by finite element simulation. The PLD deposited membranes integrated in a silicon based platform allows a drastic reduction of the thermal mass and the working temperature, shortening the sensor warm up time. The use of thin film technology diminishes the active part of the sensor, assuring low power consumption and reduced material costs.

Resume : Perovskite type (La1-xSrx)CoO3-δand the related compounds have been studied as one of the promising candidates for the cathode of intermediate temperature (500-700 oC) solid oxide fuel cells (IT-SOFCs); they show high mixed electronic/ionic conductivity and high electro-catalytic activity. Despite their advantages, the main problem in utilization of perovskite type (La1-xSrx)CoO3-δ and all other relevant derivatives in SOFCs is their insufficient oxygen reduction rate (ORR), which is mainly hindered by dissociative adsorption of oxygen at the cathode surface along with bulk ionic transport at intermediate temperatures.Especially in case of thin film cathodes, in which current loss is minimum, surface oxygen exchange becomes the source of resistance. Nonetheless, anomalously enhanced oxygen reduction kinetic around the (La1-xSrx)CoO3-δ/(La1-ySry)2CoO4 δ interface has been reported in recent years. Exploiting this interface induced ORR enhancement can decrease operation temperature of SOFC cathode and extend its chemical stability. Different approaches have been adopted in SOFC cathode construction by use of hetero interface. Since horizontal alignment of hetero interface showed blockage of enhanced ORR, there was a need to obtain vertically aligned interfaces. Cathodes with vertical interfaces were obtained by PLD using target obtained via mechanically mixing separately synthesized powders with some success [1]. This study resulted in 1000 Ωcm2 at 400 oC and 0.21 atm P_(O_2 ), which is still behind the requirements at this temperature. In the prsent work, (La0,8Sr0,2)CoO3-δ (LSC113) and (La0,5Sr0,5)2CoO4 δ (LSC214) powders were synthesized through low temperature solution based methods, ensuring the absence of any undesired phase [2]. Two oxide sputtering targets have been obtained from these powders by pressing them into 2-inch diameter pellets. The present work attempts to achieve finely mixed interfaces via co-sputtering of LSC113 and LSC214 targets. A magnetron sputtering technique is usedto obtain thin film cathodeswhere the relative amounts of (La0,8Sr0,2)CoO3-δ and (La0,5Sr0,5)2CoO4 δ phases varied. This was achievedby placing the substrates ((Ce0.9Gd0.1)O2-x) in a position whose distance varied with respect to sputtering targets. In this way more than ten different films are coated for ORR measurement. Consequently thin film half cells having submicron thicknesses were characterized via in-situ potentiostatic electrochemical impedance spectroscopy (EIS) with a frequency range between 1MHz-10mHz and 10 mV perturbation voltage amplitude. In-situ EIS measurements are conducted under different oxygen partial pressure and temperature combinations. The study determining the surface oxygen exchange constant from EIS measurements aims to identify conditions giving the highest density of interfaces, the best associated ORR and hence the lowest possible area specific resistance at 400 oC which is critical threshold for surface Sr segregation. Acknowledgement: The work reported in this study was supported by ERAFRICA FP7 Program: No. RE-037 HENERGY –TUBİTAK Project 114M128. References: [1] Ma, W., Kim, J. J., Tsvetkov, N., Daio, T., Kuru, Y., Cai, Z., et al. (2015). Vertically aligned nanocomposite La0.8Sr0.2CoO3/(La0.5Sr0.5)2CoO4 cathodes-electronic structure surface chemistry and oxygen reduction kinetics. Journal of Material Chemistry A (3), 207-219. [2]Torunoğlu Z. Ç., Demircan O., Kalay Y. E., Öztürk T. and Kuru Y., Utilization of Enhanced Oxygen Reduction Rate at Hetero Interface of(La0,8Sr0,2)CoO3-δ/(La0,5Sr0,5)2CoO4 δvia Dual Phase Synthesis. International Symposium on Materials for Energy Storage and Conversion Ankara, Turkey.Oral Presentation, September 7-9, 2015

Resume : New electrolytes are needed in lithium-based battery research in order to increase both safety and electrochemical performance. The mixture of an ionic liquid with a lithium salt represents a conceptually new class of electrolytes for high-temperature lithium batteries, termed “ionic liquid-in-salt” [1]. We used 7Li NMR spectroscopy, see, e.g., [2], to study both local electronic structures and Li self-diffu¬sion in LiTFSI and LixEMIM(1–x)TFSI with x = 0.9. The NMR spectra, recorded under static conditions, perfectly agree with the results from differential scanning calorimetry. Upon heating to 513 K they clearly reveal several double phase regions; the known solid-state phase transformation of LiTFSI can be well recognized by the change of the quadruple powder pattern of the 7Li NMR spectra of LiTFSI. A rapid increase in long-range ion conductivity, within two orders of magnitudes, takes place when the 1/2 EMIMTFSI/LiTFSI phase starts to melt. This can also be monitored by temperature-variable 7Li spin-lattice relaxation (SLR) NMR. If recorded up to delay times of 1000 s, the pronounced bi-exponential 7Li SLR NMR transients found directly reveal a subset of highly mobile Li ions, partly identified as [Li(TFSI)2]-, which can be well discriminated from the response of pure LiTFSI. Most likely, this Li sub-ensemble, which is anticipated to be located at the LiTFSI:EMIMTFSI interfacial regions, is responsible for the enhanced ion conductivity observed. [1] Marczewski M., Stanje B., Hanzu I., Wilkening M., Johansson P.; Phys. Chem. Chem. Phys., 2014,16, 12341-12349 [2] Stanje B., Epp V., Nakhal S., Lerch M., Wilkening M.; ACS Appl. Mater. Interfaces, 2015, 7 (7), pp 4089–4099

Resume : A rapid increase of the demand on efficient energy storage solutions requires new approaches beyond the Li-ion technology. In particular, metal-air batteries as well as solid-state fuel cells offer a great potential for high-energy-density storage devices. Since the efficiency of such devices is significantly limited by the activation of both oxygen reduction reaction (ORR) and ionic and electronic conductivities, an adequate porosity as well as a controlled doping are required. Ion implantation is a key technology to achieve this goal. In this work, p- and n-doped strontium titanate (SrTiO3) single crystals were used as oxide materials. The oxygen exchange kinetics as well as the structural changes of the SrTiO3 crystal surface induced by the ion implantation were investigated. On the one hand, the depth profile of dopant concentration and dopant valence state were determined using sputtered X-ray photoelectron spectroscopy (XPS). On the other hand, the overall oxygen exchange kinetic of the implanted SrTiO3 crystal was quantitatively described by means of coulometric titration using Zirox (Zirox GmbH, Germany) system. Furthermore, the surface morphology of the samples was investigated using atomic force microscopy (AFM). It was shown that a change of surface oxygen exchange rate of the doped single crystals is dependent on the implanted ion species and their fluence, respectively. In addition, the implanted layers remain stable even at temperatures well beyond the usual operation temperature of solid-oxide fuel cells.

Resume : Solid oxide fuel cell (SOFC) is one of the most promising next-generation energy conversion devices having high efficiency. In SOFC, a mixed ionic-electronic conducting (MIEC) oxide is typically used as a cathode material. The electrochemical reaction in a porous MIEC electrode takes place not only at the electrode/electrolyte/gas interface (triple phase boundary) but also on the surfaces of the oxide particles. Thus, the area where the electrode reaction occurs is considered to expand to a certain distance from the electrode/electrolyte interface. For the development of high performance SOFC cathode, it is important to understand how such a reaction distribution was formed. Many numerical simulations have been carried out to estimate the reaction distribution in SOFC electrodes. However, to our best knowledge, only a few experimental evaluations have been done so far. From the backgrounds mentioned above, in this work, we aimed to experimentally evaluate the reaction distribution in an SOFC MIEC cathode. La0.6Sr0.4CoO3-δ and La0.6Sr0.4Co0.2Fe0.8O3-δ were chosen as typical mixed conducting oxides. For the quantitative evaluation, a patterned thin film electrode, which was a kind of a columnar electrode modeling the porous electrode, was fabricated by means of the lithographic technique. Electrochemical impedance spectroscopy (EIS) measurements were performed with/without cathodic bias at various temperatures and oxygen partial pressures. The reaction distribution was evaluated by fitting the spectra with a Gerischer-type equivalent circuit, and the factors controlling the reaction distribution were discussed.

Resume : The search for suitable mixed ion-electron conducting cathode materials for intermediate temperature solid oxide fuel cells has led to the study of complex oxide perovskite materials such as LaCoO3 doped with Sr and Fe. Understanding the role of defects, both intrinsic and extrinsic, is key to understanding the transport properties of this system. We present results of first principles calculations on the formation of intrinsic point defects, substitutional defects, and defect complexes in LaCoO3. From our results we derive the most favourable charge compensation mechanisms for the defects studied, whether via point defect or polaron formation, having implications for the ionic and electronic transport mechanisms that dominate in the system. Our results agree well with available experiment and provide design strategies for fuel cell cathode applications.

Resume : Perovskite structured transition metal oxides are of fundamental and technological importance due to a wide range of physicochemical properties stemming from the interactions between the transition metals and anions. Hydride ions (H-) have been shown to incorporate into BaTiO3 and induce a remarkable change in its electronic and transport properties. In this contribution, we elaborate on the behaviour of hydride ions in BaTiO3, and their interactions with electron polarons from first principles calculations. Our calculations demonstrate that the hydride anions induce a localized in-gap state in BaTiO3-xHx. The in-gap state is localized at 0.95 eV below the conduction band, and is responsible for experimental observations of semi-conduction and blue colour of BaTiO3-xHx. The oppositely charged hydration ions and electron polarons are found to strongly interact with each other, forming a complex with a binding energy of 0.24 eV. Experiments have shown that hydride ions in BaTiO3 readily may be exchanged with surrounding gasses, indicating of fast hydride ion transport. Our calculations show that this fast transport stems from a oxygen-vacancy mediated mechanism in which the hydride ions migrate to adjacent oxygen vacancies, characterized by a barrier of merely 0.28 eV. However, low oxygen vacancy concentration would inhibit long range hydride transport. Our calculations provide a valuable understanding on hydride transport behaviour, and also pave the way for investigation of photoadsorption in other metal oxides.

Resume : Two-step thermochemical fuel production is an attractive renewable energy technology that utilizes solar energy and converts CO2 and water directly to syngas. The success of the technology is predicated on the discovery of materials with a high oxygen release and favorable thermodynamics for water and CO2- splitting. Perovskites with the general formula ABO3-d have emerged as a promising material class with wide possibilities in doping both the A- and the B-site cation. By a systematic search of thermodynamics and oxygen release characteristics in perovskites, we have synthesized Co- and Sr-doped lanthanum chromates. With this composition, we tune the thermodynamics of vacancy formation towards the Gibbs free energy change of reduction of water and CO2. Utilizing in-situ X-ray diffraction, Raman spectroscopy, and electron microscopy it is shown that the material is chemically stable under relevant conditions for solar-to-fuel conversion. We report an optimum in doping concentration of chromium in the perovskite, for which La0.6Sr0.4Cr1-xCoxO3-δ, has a fuel yield 25 times higher than state-of-the-art material ceria when cycling at 800-1200˚C, or similar performance to ceria operated between 1000-1500˚C. Accessing this new low temperature range by the smart material choice is highly beneficial for solar-to-fuel reactors in terms of lowered heat loss, efficiency, and material requirements. References: 1) A. Bork, M. Kubicek, M Struzik, J. Rupp. J Mater Chem A, 3, 15546 (2015)

Resume : Yttrium doped barium zirconates (BZY) are considered for use in protonic ceramic fuel cells (PCFCs) and electrolyzers (PCECs) where some portion of the electrolyte is in contact with oxidizing atmosphere at elevated temperature. The performance of these devices depends on the knowledge of the resulting defect concentrations, which are functions of oxygen and water vapor pressures. BZY has three mobile charged defects under moist oxidizing conditions: oxygen ion vacancies, protonic defects and electronic defects (polarons on the oxygen site). There is a discrepancy in the literature whether the oxidation reaction (where oxygen dissociates into oxygen vacancies to form polarons on the oxygen site) is exothermic or endothermic. In this work, we present thermogravimetric analysis on a BZY10 (BaZr0.9Y0.1O3-d) rod in various oxidizing atmospheres in the temperature range 900-300 °C. This kind of measurements is usually tricky because of the very small weight changes. Reliable data could be obtained using a large hanging rod. The weight gain from the oxygen incorporation into the BZY lattice increases with decreasing temperature, confirming the exothermicity of the oxidation reaction.

Resume : Some Ni-based perovskites have the ability to exsolute Ni nanoparticles at their surface in reductive conditions leading for instance to potentialities as hydrogen electrode for SOFC [1], or enhancement of their catalytic properties [2]. Ba8Ta6NiO24 is a complex perovskite-like oxide [3] that exhibits pairs of octahedrons sharing faces. The later are composed of two lacunar () distinct sites, one partially occupied by Ta5 / and the other with a mixed occupancy Ni2 /Ta5 / which offer great possibilities of insertion and substitution. This particular structure leads for instance to good performances in the field of microwave dielectrics [4]. We could achieve on Ba8Ta6NiO24 the topotactic exsolution of nanoparticles of Nickel at its surface without destruction of the parent oxide. The partial Ni exsolution from the face shared octahedrons leads to rearrangement of the cations/ within this entity. We propose an exsolution mechanism and correlate this phenomenon with the properties of this phase. In particular, its catalytic activity toward DRM will be discussed. In the family BaM2X2O8, (M= Fe, Co, Ni and X= P, V, As) which layered structure exhibits a honeycomb lattice, oxidative exsolution of Fe into Fe2O3 nanoparticles at the surface occurs and was fully investigated [5]. Recently we observed Ni nanoparticles reductive exsolution in particular conditions using ammonia. It is accompanied by a small amount of nitrogen exchanging oxygen in the honeycomb layer. A complex rearrangement takes place in the later while the surface of the solid is decorated by nano-Ni. The behavior of the so-formed solid is peculiar with temperature in reductive atmosphere and was studied in the TEM. Similarly than Ba8Ta6NiO24, the exsolution process is fully reversible upon heating in air. Investigation of the properties of this phase will be also presented. [1] C. Arrivé et al., Journal of Power Sources 223 (2013) 341 - 348 [2] B.H. Park et al., Solid State Ionics 262 (2014) 345-348 [3] Abakumov A. M. et al., J. Solid State Chem., 125, 102-107 (1996), Article N°. 0270 [4] S. Kawaguchi et al., J. Eur. Ceram. Soc. 26 (2006) 2045-2049 [5] Angew. Chem. Int. Ed. 2014, 53, 13365 3365 8. Inorg. Chem., 2015, 54 (17), pp 87333, 13. Cryst. Growth Des. 2015, 15, 42375, 15

Resume : Solid-state batteries are expected to solve problems originated from non-aqueous liquid electrolytes in lithium-ion batteries. Although they had been suffering from low power densities due to low ionic conductivities of solid electrolytes, conductivities comparable to liquid systems have been achieved in some recently-developed solid electrolytes, and the highest conductivity has reached 10$^{−2}$ S cm$^{−1}$ among sulfides. Consequently, ion transport in solid electrolytes can cease from rate-determining in sulfide electrolyte systems; however, it has been replaced by that across interfaces. Ionic conductors often exhibit anomalous ionic conduction in space-charge layers formed at their surface or interface, which is categorized as “nanoionics”. The sulfide solid electrolytes have not improved power density of solid-state batteries in spite of the high ionic conductivities, which is attributed to the space-charge layer appearing as a lithium-depleted layer at the high-voltage cathode/solid electrolyte interface. A buffer layer interposed at the interface in order to suppress the lithium depletion dramatically improved the power density. These interface phenomena can be explained on the basis of classical Nernst equation and the first-principles calculation.

Resume : Lithium-ion batteries presents safety hazards due to their electrolyte. The electrolyte is typically an organic flammable liquid, and their proximity to an oxidizing agent can lead to thermal runaway and explosion in extreme cases. To overcome those inconveniences, the use of solid state electrolyte has been suggested. They are inflammable and exhibit a higher stability. However, their low ionic conductivity (less than 10^-4 S cm-1) hindered so far their use in commercial batteries. LiZr2(PO4)3 in the NASICON structure is a promising solid state electrolyte with attractive Li-ion conductivity of 10^-4 S cm-1 and stability. This compound undergoes a reversible phase transition from a triclinic structure (called alpha') to a rhomboedral one (alpha phase) at 330K. It is experimentally known that the conductivity of the alpha' phase is three orders of magnitude lower than the estimated conductivity of the rhomboedral phase at room temperature. This motivated strategies to stabilize the high temperature alpha phase at room temperature by incorporation of Ca for instance. It is however not clear yet to what atomistic mechanism could explain this large difference between two similar structure: the alpha and alpha’ phases. In this work, we have performed an ab initio study of Li diffusion in both alpha and alpha’ phases using ab initio molecular dynamics and nudge elastic band. Based on this computational insight, we identify the differences and similarity between the two phases and shine light on the reason for the higher performance of the alpha phase.

Resume : In the quest for low-cost large-scale energy storage systems, sodium ion batteries (SIBs) are considered as a promising candidate. Notable examples of Na-based cathode materials based on earth-abundant elements and showing both high voltages, fast rate capability notable examples include Na2+2xFe2-x(SO4)3 and Na2FeP2O7. The rate performance of such new insertion cathode materials can be predicted from their structure by modelling the energetics of Na+ pathways factoring in the effect of structural changes during charging/discharging on the Na site energies. The achievable capacity at high rates is typically limited by the kinetics of sodium insertion and extraction. To understand and overcome such limitations, it is important to look at the energetics of the formation and interaction of the mobile defects at various stages during charging/discharging. Here we report a computational study on SIB cathode materials for based on the bond valence site energy. As we recently exemplified for alluaudite-type Na2+2xFe2-x(SO4)3,[1]defect interactions and their influence on the overall sodium ion transport were investigated in realistic local structure models representing different stages of sodium extraction. Based on these results it is discussed how the pathway characteristics accessible by such a computationally cheap tool allow us to predict the rate performance of electrode materials. [1] L. L. Wong, H. M. Chen, & S. Adams, Phys. Chem. Chem. Phys. 2015, 17, 9186.

Resume : Understanding and controlling the defect interactions and transport properties of Li ions in Si anodes is crucial for developing high-performance Li-ion batteries. With a theoretical capacity of ~4140 mAhg-1, which is more than 10 times over that of graphite, the practical use of Si anodes is, however, hampered by capacity fading and volume changes that occur during cycling. Deeper insights into mechanisms of Li-Si reactions are critically needed to address this problem. In this work, we report on Li-ion implantation into thin amorphous and single crystalline <100>-oriented Si membranes using a focused Li-ion beam (LiFIB). The scanning LiFIB boasts a probe size of a few tens of nm at energies ranging from 500 eV to 6 keV and beam currents of a few pA, allowing one to implant directly Li+ cations into any material with nanoscale precision. We characterize Li-Si interactions and lateral distributions of Li in well-defined implanted regions with a combination of low-energy ion-induced secondary electron LiFIB imaging and high-resolution analytical scanning/transmission electron microscopy. Our findings indicate that nanoscale processes that occur during Li+ implantation involve a series of transformations, including amorphization of the Si matrix, clustering of interstitials in tetragonal sites of the matrix, creation of interstitial-rich extended stepped and v-shaped defects, and concentration gradient driven diffusion of Li ions followed by the formation of Li-Si phases.

Resume : Gouy and Chapman developed their theory of the diffuse electrochemical double layer more than a hundred years ago. That this theory — which combined the differential form of Gauss’s law with Boltzmann statistics and found a closed-form solution for the resulting Poisson-Boltzmann equation — and its descendants should still provide the bulk of our understanding of the relationship between the properties of interfaces and those of the bulk electrolyte is testament to its insight and power. Although Poisson-Boltzmann theory is strictly valid only in the limit of infinite dilution, its applications have long ventured well outside of the dilute range. However, a growing number of experiments reveal that concentration profiles of ions near interfaces in concentrated solutions do not fit the predictions of Poisson-Boltzmann theory. Poisson-Cahn theory replaces the Boltzmann chemical potential with an expression derived from the Cahn-Hilliard miscibility theory, taking both defect interactions and gradient effects into account. The resulting theory is not closed-form, but its predictions match the experimental evidence from concentrated systems much more faithfully. Applications to be presented include the oxygen-exposed surfaces of perovskite mixed conductors as well as grain boundaries in doped ceria.

Resume : Ceria (CeO2) and yttria-stabilized zirconia (YSZ) are two promising electrolyte materials used in solid oxide fuel cells due to their high ion conductivities. Theory predicts that the formation energy of oxygen vacancies is considerably decreased at free surfaces and internal interfaces. Oxygen vacancies are expected to segregate at the interfaces and thus might provide an easy path for rapid ion conduction. The aim of the present work is to obtain insights into the structure and chemistry of interfaces between ceria films and YSZ substrates. Analytical scanning transmission electron microscopy (STEM) is the method of choice for a comprehensive materials characterization in terms of structure and chemistry down to atomic levels. In the present study high-resolution structural and electron spectroscopic imaging (ESI) was performed in an advanced probe-corrected TEM/STEM system (JEOL JEM-ARM200F). Electron energy-loss spectroscopy (EELS) in STEM enables to probe the local oxidation state of Ce ions in interface regions by utilizing valence sensitive features in the energy-loss near-edge fine structures (ELNES) of Ce-M4,5 edges. Continuous epitaxial films of pure and Gd-doped ceria were grown by pulsed laser deposition respectively on (111) and (100) YSZ substrates showing a cube-on-cube orientation relationship. No reaction layers or secondary phases were identified at the interfaces. The film-substrate lattice mismatch of ca. 5.3% is accommodated by periodically spaced misfit dislocations observed at the interface with extra atomic planes in the YSZ substrate [1]. EELS line scans across the interface clearly reveal a reduction of tetravalent to trivalent Ce in narrow interface regions by utilizing valence sensitive Ce-M4,5 ELNES features. Atomic column-resolved ESI were acquired across whole ceria films including interface and surface regions and in selected regions of interest across ceria-YSZ interfaces. Multiple linear least-squares (MLLS) fitting enables an unambiguous visualization of the tetravalent to trivalent Ce reduction at interfaces and free surfaces [1]. In both, Gd-doped ceria films with up to 10 at.% Gd and undoped samples, a reduced interface region of about 1.2 ± 0.1 nm width was observed. Assuming charge balance, the presence of trivalent Ce ions is seen as evidence of oxygen vacancy formation in narrow interface regions. In summary, it is concluded that advanced analytical TEM/STEM methods enable the elucidation of local non-stoichiometry, which is crucial not only for understanding charge transport mechanisms in these hetero-structured materials, but also for understanding the catalytic properties of ceria [2]. References [1] K. Song et al.: APL Materials 2 (2014) 032104 [2] The research leading to these results has received funding from the European Union Seventh Framework Program [FP/2007-2013] under grant agreement no 312483 (ESTEEM2).

Resume : Yttrium-doped barium zirconate (BaZr1-xYxO3-d, BZY) is the most studied high-temperature proton conductor because of its stability in CO2 and H2O containing atmospheres and its satisfactory bulk conductivity. However, the development of BZY-based devices is hindered by the presence of resistive grain boundaries and the difficulty of preparing dense thin membranes. The blocking character of the grain boundaries has been explained by the space-charge layer model, with a positively charged grain-boundary core and a surrounding depletion layer for protons. The validity of this model has only been proven by indirect methods such as conductivity measurements under a bias or as a function of the oxygen partial pressure. In this work, atom probe tomography is used to get the chemical quantification of grain boundaries of several BZY specimens, prepared by different processes. We can show, for the first time, a direct proof of the positively charged grain-boundary core with the accumulation of oxide-ion vacancies. The results also highlight the significant impact of the fabrication process on the resistive behavior of the BZY grain boundaries, which can have largely variable chemistries. Among these processes, solid-state reactive sintering offers the best advantages: it leads to a large-grained microstructure with the least resistive grain boundaries, whilst being industrially viable.

Resume : Electronic states at grain boundaries can trap charges and lead to potential gradients. Typical barrier heights are of the order of a hundred meV in polycrystalline semiconducting oxide thin films like In2O3. The barrier heights can be determined using temperature dependent Hall effect measurements. We have developed an approach to quantify the density of electronic states at grain boundaries and the associated charge neutrality level by comparing measurements of barrier heights in dependence of carrier concentration with numerical solutions of the Poisson equation. The analyis requires a variation of the carrier concentration of a single film over several orders of magnitude. In addition, surface contributions to the electrical conductivity have to be ruled out. Both is achieved using a home made Hall effect relaxation setup but annealing a particular sample in controlled atmosphere. The approach provides a unique experimental access to quantities, which can otherwise only be addressed using electronic structure calculations.

Resume : Resistance random access memory (ReRAM), which utilizes two or more resistive states of a material system for data storage, has attracted considerable attention as a future non-volatile memory concept. It has become widely accepted that resistive switching in oxides is in most cases connected with a voltage-driven oxygen vacancy movement and a resulting redox process. However, the current knowledge of the microscopic details of the redox-processes is very limited. One of the obstacles for its further elucidation is that the net changes of structure, stoichiometry and valence state during electroforming and switching are very small and occur primarily at the electrode interface or within nanoscale filaments. By employing different approaches of X-ray based spectromicroscopy, we could prove the formation of an oxygen vacancy enriched filament in epitaxial SrTiO3 thin film devices, which occurs preferentially at preformed positions such as extended defects. For high current operation, electroforming goes along with the formation of SrO at the electrode interface which has a significant impact on the stabilization of the low resistance state of the devices. In operando studies of SrTiO3 devices with photoelectron-transparent graphene electrodes enabled us to detect reversible changes of the O K-edge spectra within spatially confined regions of the devices. Based on these results, we obtain a quantitative estimate of the amount of oxygen vacancies shifted during the switching process.

Resume : It has previously been demonstrated that La0.5Sr0.5Co0.5Mn0.5O3-δ (LSMC) can accommodate both a fully oxidised (δ = 0) and a reduced phase (δ = 0.62), making it one of the few reported perovskite oxides that is stable with δ > 0.5 [1]. Understanding reversible redox processes in perovskites, and the extent to which this is possible in LSMC, is expected to be critical in developing memristors. Epitaxial thin films of LSMC (15 – 40 nm) were grown on conductive, single crystal SrTiO3 with 0.5wt% Nb-doping using pulsed laser deposition. The films were isotopically labelled with 18O by oxygen exchange at 200 mbar and intermediate temperatures (400 – 500°C). Secondary ion mass spectrometry (SIMS) depth profiling confirmed an enriched 18O isotopic fraction of 30-45%. In situ application of bias (0 – 25 V), inside the SIMS high vacuum chamber, through the film resulted in a switching event with a change in resistance of two orders of magnitude (6MΩ to 30kΩ). This switching event resulted in 18O depletion at the point of the top electrode contact. Using sequential SIMS mapping of 18O, we have been able to track a filament through the film. Initial results under ambient conditions suggest that reversible switching can be achieved. These results are crucial for the investigation of oxygen ion transport in perovskites as a result of applied bias, and could lead to the development of new solid state memristors. [1] Aguadero, A. et al. Angew. Chem. Int. Ed. Engl. 50, 6557–61 (2011).

Resume : Within energy and information technologies the development of functional materials with improved ionic and electronic transport properties dominates the innovation in many relevant technological fields as: fuel cells, batteries, sensors and memristive devices. Whilst depending on the technology the approach varies, in every case the electrochemical performance of the different devices is strongly affected by defect nature and concentration due to their intrinsic relation with charge carrier transport processes. Consequently, complex oxide thin film materials, where the concentration of point and extended defects is especially high, are becoming increasingly important in these applications. This work evaluates the electrochemical properties of mixed conducting La0.8Sr0.2Mn1-xCoxO3±δ (x = 0, 0.25 and 0.5) perovskite thin films. Samples were fabricated by combinatorial Pulsed Laser Deposition (PLD) as a function of Co concentration. Films with different oxygen stoichiometry were obtained by post-deposition annealing. For a better understanding of the structure-activity relationships physicochemical analyses were carried out to support resistive switching findings from electrical measurements, comprising 4-point Van der Pauw, cyclic voltammetry and pulsed tests. In general, undoped films presented a small current-voltage hysteresis while 50 mol. % Co-substituted samples showed the largest memristive response. For these samples, pulsed measurements revealed multilevel resistance states which allow tuning the resistance state based on the magnitude and duration of an applied voltage pulse. Stable and reproducible non-volatile high resistance and low resistance states were also achieved.

Resume : The migration of ions in amorphous HfOx was studied by means of molecular dynamics simulations. The calculations are based on an empirical Born-Mayer potential that was tested in detail to reproduce the thermal expansion, the heat capacity as well as diffusion properties for the doped cubic and monoclinic phases. Stoichiometric and oxygen deficient amorphous HfOx (i.e. x = 2 and x <2) supercells were generated via a melt and quench procedure and analysed in the temperature range of 875 K< T/K < 2000 K. The evolution of the mean square displacement as a function of diffusion time indicated a non-linear, sub-diffusive behaviour for both types of ions. Diffusion was also found for non-defective simulation cells, further emphasising that the crystalline model of vacancy mediated migration cannot be applied for amorphous HfO2. Consequently, we introduced a new approach to determine possible activation barrier distributions for oxygen ion migration based on the residence times of the ions. It reveals a range of activation energies/enthalpies from 0.3 eV up to 0.83 eV, while the cubic crystalline cells showed 0.43 eV. Concluding, we present surprising insights into diffusion in amorphous HfO2 and propose a picture of diffusion mediated by the local ionic environment combined with a new simple approach to estimate the distribution of activation barriers during the diffusion process.

Resume : Nanoionic-based memristive devices have been classified into two modes, electrochemical metallization memories (ECM) and valence change memories (VCM), based on the fundamental switching mechanisms. In this work, we investigated Ag/MoO3-x-based memristive devices, which show a normal ECM switching mode after an electroforming process. While, in the lower voltage sweep range, the resistance switching behavior changed to VCM mode with the opposite switching polarity to the ECM mode. By current-voltage measurements under different ambient atmospheres and x-ray photoemission spectroscopy analysis, the electrochemical anodic passivation of Ag electrode and valence change of molybdenum ions during resistance switching have been demonstrated. The crucial role of moisture adsorption in the switching mode transition has been clarified. These results provide the fundamental insight necessary to the material selection, film structure, configuration design, and switching mechanism modeling for engineering high-performance memristive devices.

Resume : Lattice point defects in ceramics are largely responsible for the overall electrical properties of the material, however these defects tend to move under externally applied voltages, leading to spatial heterogeneities in stoichiometry and conductivity. When the electrodes are impermeable to mass transport, point defects accumulate in the near-electrode region and can be responsible for the time-dependent changes in the leakage current observed in many devices. Often these types of phenomena control device reliability and lifetime. This talk will focus the influence of the electrochemical boundary conditions on the spatio-temporal redistribution of point defects and the concomitant effects on conductivity. Particular attention will be given to recent experimental studies on single-crystal TiO2, SrTiO3 and BaTiO3. A combination of electrical transport and electron microscopy analyses help provide insight into the interface evolution from the atomistic to micrometer length scale. This work was sponsored by the National Science Foundation under grant DMR-1132058 and AFOSR under grant FA9550-14-1-0067.

Resume : The structure, energetics and transport properties of TiO2 anatase with different densities of oxygen vacancies are studied by computer simulations using a variety of techniques ranging from first-principles to Monte Carlo methods. This investigation is motivated by the recent development of memristive electronic devices, usually made of metal oxides in which arrays of defects control the resistance switching mechanisms. Anatase, in particular, emerged as one of the most promising candidate for memristor design. However, the microscopic behavior of these multi-vacancy systems is not yet entirely understood. This work investigates electronic and transport properties of TiO2 anatase structures containing from one to several oxygen vacancies both in neutral and charged states by adding an Hubbard-like term (U) to the the Generalized Gradient approximation to the electron density (GGA+U). Calculated observables are the formation energy of oxygen defects, vacancy cohesion energies and Local Density of States for different vacancy distributions. Furthermore, to demonstrate the correspondence between energetically stable states and the conductive phase of memristors, electron transport is studied first via the integrated local density of states (ILDOS), and then within a tight-binding approximation to density functional theory. Finally, a Kinetic Mote Carlo model of the conductive channel formation in bulk anatase, based on the nudged-elastic band (NEB) calculations of the vacancy diffusion rates, is reported.

Resume : Large forming voltages required for realizing stable resistive switching in hafnium oxide (HfO2) based resistive random access memory (RRAM) devices can lead to variations in switching behavior from device to device decreasing the device yield. In turn, efforts have been made towards improving yield by oxygen engineering and doping of HfO2 thin films to stabilize oxygen vacancy concentrations far beyond the thermodynamical equilibrium. The conductivity of HfO2-x films grown by MBE could be tuned in a wide range by varying the oxygen flow, in turn, creating a defect band at the Fermi-level [1,2]. In the RRAM devices, HfO2 typically crystallized in a monoclinic symmetry (m-HfO2), whereas the oxygen deficient films showed oxygen vacancy stabilized tetragonal like phase of hafnium oxide (t-HfOx), as verified by X-ray diffraction [2]. A traditional doping approach in HfO2 (La, Ta) was also used to stabilize defect induced higher symmetry phases. The forming voltages and its thickness dependence was found to be largely suppressed for highly oxygen deficient films paving the way for low power devices in future. [3] A comparison of HfO2 based RRAM devices with varying dopant and crystallinity also showed tunable forming voltage trends and higher ratios due to improved stability of the resistance states. [1] E. Hildebrandt, et al. J. Appl. Phys. 112, 114112 (2012). [2] S. U. Sharath et al., Appl. Phys. Lett. 104, 063502 (2014). [3] S. U. Sharath et al., Appl. Phys. Lett., 105, 073505 (2014).

Resume : Recently, resistive random access memory (RRAM) has gained a lot of attention due to its promising properties: fast switching times, high endurance, and low power consumption. RRAM devices are non-volatile memories (NVM) based on switching between a low resistance state (LRS) and a high resistance state (HRS) by conducting filaments (CF) which are formed and disrupted by applying voltages of different polarities. RRAM devices usually have a simple metal-insulator-metal stack (MIM) structure. Insulating transition metal oxides like hafnium oxide (HfO2) are promising for embedded RRAM due to its established CMOS compatibility. In-situ stacks of TiN/HfO𝑥 with oxygen deficient stoichiometry were deposited by molecular beam epitaxy (MBE) using radical sources with different gases (oxygen and nitrogen) [1]. Device stacks of Pt/HfO𝑥/TiN with varying device areas were patterned using lithography. Since the forming process of the filament is crucial for good device endurance and still not fully understood, the temperature of formation was varied to investigate its influence on the forming process and the endurance. The effect of Al2O3 interlayers acting as oxygen diffusion barriers on switching performance has also been investigated. Therefore stacks with and without alumina interlayer have been characterized and compared. [1] S. U. Sharath et al., Appl. Phys. Lett. 104, 063502 (2014).

Resume : Protons dissolve in oxides from surrounding water vapor or hydrogen, forming hydroxide defects, and leading to proton conduction of varying magnitude and significance. The protons migrate predominantly by activated jumps between the comparatively stationary host oxide ions. This transport depends on the dynamics of the oxide ion sublattice, and is generally of relatively low activation energy. However, the protons easily become associated with other defects, leading to trapping, increased apparent activation energies, and lower proton conductivity. The protons are typically trapped to acceptor dopants in the nearest or next nearest positions to the host oxide ion, and it is recently suggested that this may lead to enhanced percolation conductivity. It is also interesting that acceptor-acceptor oxygen vacancy triplets may be formed during sintering and other thermal treatments and that these may not be hydrated like free oxygen vacancies, and that this may explain large differences in proton levels between samples. In addition to proton-acceptor interactions, the presentation will touch upon a range interactions between protons and other defects in oxides, e.g. other protons, electrons, cation vacancies, oxide interstitials. Proton defect interactions may be studied by a range of experimental methods, and computer simulation are today providing insight of high predictive power, both with respect to the defect structure thermodynamics and defect mobilities.

Resume : Acceptor dopants are key ingredients in functional oxides, inducing charge-compensating defects such as oxygen vacancies and protons ? and thus ionic conductivity. In the present contribution, we combine experimental and computational techniques to address the crucial role of acceptor and oxygen vacancy clustering on the hydration thermodynamics and transport properties in acceptor doped barium cerate. Thermogravimetric and impedance spectroscopy measurements were conducted to determine hydration thermodynamics and transport properties, respectively, while DFT calculations provided the energetics and stability of defects and defect clusters. The results herein demonstrate that clusters of acceptors and oxygen vacancies can dominate the defect structure. These cluster configurations will only form during synthesis and their concentrations are frozen-in at lower temperatures. Consequently, the concentration of acceptor and oxygen vacancy clusters is strongly related to the synthesis temperature employed. This in turn results in major differences in the hydration and transport properties where compositions display higher proton mobilities when the synthesis temperature is raised. The formation and stability of acceptor and oxygen vacancy clusters for different compositions are addressed and the effects on the hydration thermodynamics and transport properties are discussed.

Resume : Thin, dense membranes of electrode-supported BaZr(0.8)Ce(0.1)Y(0.1)O3-d, BZCY81, may now be fabricated with relative ease at commercial scale. Devices with these membranes have potential for supporting the emerging hydrogen economy and reducing dependence on fossil fuels. With protonic ceramic electrolytes it is possible to galvanically transport pure hydrogen from one side of a membrane to the other, making it possible to fabricate electrochemical devices and systems that were previously impractical or impossible. However, proton conduction requires hydration of the ceramic membrane by the Stotz-Wagner mechanism, where the extent of hydration depends on local pH2O in a complex way. In a protonic ceramic membrane reformer, for example, it is necessary to transport hydrogen from the feed side, where high pH2O is maintained, to the permeate side, with low pH2O. This hydration gradient introduces a non-trivial term in electrochemical potential that must be accounted for in a galvanic device. The theoretical basis and experimental measurements for a BZCY81 membrane operating under asymmetric steam conditions at open circuit will be discussed. The complex interaction between ambipolar steam permeation and the Nernst potential due to pH2 and pH2O will be addressed.

Resume : Enhancement of ionic conductivity in thin films or multilayers of oxide materials, i.e. doped zirconia and ceria, has sparked great interest in the search for fast ion conducting structures for fuel cells as well as for red-ox based resistive memories. The enhancement in ionic conductivity in such structures could be attributed to elastic strain arising from the lattice mismatch at the interface. However, this assumes that the interface between two materials is perfectly coherent, while in most cases dislocations are observed, and these dislocations relax the interfacial elastic strain. The strain field and the electrostatic field that arises from the dislocations can also impact the defect stability, distribution and mobility in these materials; and yet, the role of dislocation on the ionic conductivity is not consistently reported in the literature nor is it clearly understood. The aim of our work is to quantitatively assess the dislocation's influence on the ionic conductivity in fluorite and perovskite oxides, exemplified by doped CeO2 and SrTiO3, respectively. Edge dislocations in in these materials are studied by atomistic simulations combining the Monte Carlo, Molecular Dynamics and Nudge Elastic Band calculations. Asymmetric distribution profiles of dopant cations and oxygen vacancies are found as a result of the strain field of the dislocation, giving rise to counterintuitive results for diffusion kinetics along and across the dislocations. The studied dislocations in such highly doped or reducible oxides trap oxygen vacancies and reduce the oxide ion mobility along the dislocation, as opposed to the fast pathways that dislocations present for atomic diffusion in metals. As a result, the potential role of elastic strain and dislocations will be contrasted in oxide ion diffusion in these reducible or doped oxides.

Resume : A number of experimental studies were published dealing on ionic trans­port in micro-/nanoscaled thin films and multilayer. In case of extrinsic ionic conductors interface strain as an origin of an increased (or decreased) ionic conductivity/diffusivity along interfaces is intensely discussed. By reviewing the literature data, one observes that the magnitude of the interface effects differs unfortunately several orders of magnitude. In this study the O2− conductivity parallel to the phase boundaries in RE2O3/YSZ multilayer (RE = rare earth metal), prepared by pulsed laser deposition on (0001) Al2O3 substrates, is measured vs. layer thickness. The misfit induced interface strain can be varied by changing the rare earth metal RE (Sc, Er, Dy). The O2− conductivity of the YSZ layers is measured by impedance spectroscopy, the texture and interface strain by XRD (pole figures). The preparation conditions have a strong influence on the measurable (thickness dependent) strain effect on the interface transport. The anisotropic O2− conductivity of highly textured samples with only one azimuthal orientation variant completely superposes the strain effects. In case of samples with two azimuthal orientation variants (or a fibre texture), the interface strain effects can be detected as reported in former studies. The influence of texture, grain boundaries and crystallite size is in the same order of magnitude as the influence of interface (misfit) strain on the total transport in thin films.

Resume : All-solid state Li ion batteries are one of next-generation rechargeable batteries, which can achieve high energy density and are free from risk of use of flammable liquid electrolytes. The components of all-solid state Li ion batteries can be exposed to mechanical stresses due to volume change of electrodes followed by insertion/extraction of Li ions, and due to lattice mismatches between electrodes and electrolyte, etc. It is known that such mechanical stresses lead to the change in the thermodynamic and/or kinetic properties of battery components. However, the influence of mechanical stress on thermodynamic and/or kinetic properties is not fully understood. From the background above, in this study, we aimed to quantitatively understand the influence of mechanical stress on Li chemical potential in typical Li ion battery cathode, LiCoO2 (LCO). The variation of Li chemical potential under mechanical stress was evaluated by EMF measurements. LCO thin films were deposited on both surfaces of a solid Li ion conductor by the PLD method. This all solid state electrochemical cell was mechanically loaded in four point bending geometry. During loading, one thin film was exposed to tensile stress and another to compressive stress. The EMF between these two stressed thin films was measured. A sudden generation of EMF was observed when the mechanical stress was applied. This indicated that Li chemical potential in the thin films was changed by the mechanical stress. The results were quantitatively discussed based on the theoretical relationship between Li chemical potential and mechanical stress, which was derived from Maxwell’s relation. The expression described well the experimental results of Li chemical potential change in LCO under mechanical stress.

Resume : The proton conducting Y-doped BaZrO3 (BZY) is a promising electrolyte for solid oxide fuel cells in the intermediate temperature range (300-700°C), thus a possible alternative to oxygen ion conductors. Tensile strain has been shown to decrease the activation energy for ion hopping in oxygen ion conductors, potentially enabling to preserve good cell efficiency al lower temperatures. For proton conductors, only studies on powders exist where compressive pressure leads to an increase in activation energy. Extrapolating, tensile strain might have a similar effect as on oxygen ion conductors. To address this experimentally, heteroepitaxial thin films with single crystal bulk conductivity are fabricated by pulsed laser deposition where tensile strain is induced by a lattice mismatch. During film growth the strain is relieved through the formation or migration of dislocations. In situ, the strain evolution is investigated with a Multi-beam Optical Stress sensor while the growth mode is observed with Reflection High Energy Electron Diffraction. Different degrees of relaxation in the buffer layer or the BZY film lead to different strain values in BZY. The proton conduction is characterised with impedance spectroscopy, in particular to investigate the activation energy as a function of strain. The proton and deuteron conductivity and activation energy is compared. The isotope effect is observed in accordance with literature, confirming the protonic nature of the charge carriers.

Resume : We investigate the stability of oxygen vacancies upon biaxial strain in the (100) pseudocubic plane and the impact of the A cation radius by studying the perovskite manganites CaMnO3, SrMnO3 and BaMnO3 using first-principles calculations. Control of defect formation through epitaxial strain in functional oxides has gained increasing interest in recent years, and can serve as a new route to engineering material properties [1,2]. First-principles studies of CaMnO3 have shown that the stability of oxygen vacancies increases with biaxial tensile strain, suggesting that oxygen vacancies serve as a relaxation mechanism for tensile strain accommodation in epitaxial thin films [3]. Oxygen vacancies in CaMnO3 are charge compensated by localized electrons, leading to a reduction of the formal oxidation state of manganese. The change in ionic radius upon reduction of manganese from Mn4+ (0.54 Å) to Mn3+ (0.645 Å) causes a considerable volume expansion. While it is well established that oxygen vacancies can induce chemical expansion in perovskite oxides [4], an analogous, but reverse, effect is also found in that tensile strain reduces the formation enthalpy of oxygen vacancies in CaMnO3 [3]. Strain can be mitigated by a change of bond lengths and bond angles, or formation of point defects like vacancies and linear defects such as dislocations. In this study of epitaxial strain, we disregard dislocations. Correlations between octahedral tilt effects, tolerance factor and vacancy stability are established to provide guidelines for controlling oxygen vacancy population in perovskite manganites. Symmetry inequivalent oxygen vacancies show different stability trends upon biaxial strain, and this is strongly correlated with the tolerance factor. The tolerance factor is thus an additional degree of freedom for controlling oxygen vacancy ordering. We further investigate the interplay between oxygen vacancy diffusion barriers and epitaxial strain, and correlate this with the tolerance factor and octahedral tiling. Finally, we investigate the effect of ferroelectric instabilities on oxygen vacancy formation in SrMnO3 and BaMnO3. It has recently been established using first principles calculations that epitaxial strain induces a ferroelectric instability in SrMnO3 [5], whereas perovskite BaMnO3 already exhibits a ferroelectric instability in the absence of strain [6], both by off-centering of the magnetic Mn4+ ions. We show that the ferroelectric distortion reduces the stability of oxygen vacancies under tensile strain, as oxygen vacancy formation impedes the ferroelectric instability. [1] S.V. Kalinin and N.A. Spaldin, Science 341, 858 (2013) [2] S.V. Kalinin et al., ACS Nano 6, 10423 (2012) [3] U. Aschauer et al., Phys. Rev. B 88, 054111 (2013) [4] S. Adler, J. Am. Ceram. Soc. 84, 2117 (2001) [5] J. H. Lee and K. M. Rabe, PRL 104, 207204 (2010) [6] J. Rondinelli et al., Phys. Rev B 79, 205119 (2009)

Resume : Hydrogen is ubiquitous in electronic devices due to processing conditions and is known to both passivate the existing and induce new structural defects. Recently it has been demonstrated that atomic hydrogen can break strained Si–O bonds and induce new defects in amorphous SiO2 [1]. Here we investigate the interaction of atomic H with a-HfO2. To study the distribution of defect properties, forty defect-free a-HfO2 models with average densities of 9.6 g/cm3 were produced by molecular dynamics simulations using the force field [2] and further optimised using Density Functional Theory (DFT) calculations. DFT with non-local functional was used to model the interaction of hydrogen atoms with strained Hf–O bonds in the a-HfO2 structures. The calculations show that H atom can break strained Hf–O bonds creating structures which consist of a 5-fold coordinated Hf atom facing a hydroxyl group. This defect introduces a one-electron level at 2.4 eV below the a-HfO2 conduction gap, ranging from 1.7-2.9 eV dependent on the sample density and local environment. An unpaired spin is highly localized on Hf atoms. These results are discussed in conjunction with the experimental data using the exhaustive photo-depopulation spectroscopy of a-HfO2 samples to investigate the temperature dependence of deep electron trapping states [2]. [1] A.-M. El-Sayed, Y. Wimmer, W. Goes, et al., Phys. Rev. B 92(1) 014107 (2015) [2] F. Cerbu, O. Madia, V. Afanasiev, et al., ECS Transactions, 64 (8), 17-22 (2014)

Resume : Alpha-Al2O3 is an important refractory material which has numerous technical applications: as an in situ growing self-healing oxide scale, as a massive material and as reinforcement fibres in composites. For modelling diffusion controlled processes (creep, sintering, alpha-alumina scale growth on aluminium bearing Fe or Ni base alloys) it is necessary to study grain boundary diffusion in polycrystalline alpha-Al2O3. However, the aluminium diffusivity is difficult to measure because aluminium has no natural tracer isotope, and the only suitable radiotracer is 26Al, which raises two difficulties. First, 26Al is artificial and causes very high production costs, and second, it has a half-life time of 7.4E+5 years and consequently a very low specific activity, which makes it difficult to apply classical radiotracer methods. In this work Secondary Ion Mass Spectrometry (SIMS) is applied to measure 26Al depth profiles resulting from diffusion experiments. This avoids the problems related to the radioactivity measurement, reduces the necessary amount of 26Al per experiment considerably, and yields a much higher spatial resolution.

Resume : Metal-oxide-based resistive switches offer new perspectives as non-volatile memories, being a promising alternative to transistor-based devices. It is known from literature that electrical properties of oxides are strongly influenced by microstructural defects. Despite the knowledge, material and engineering guidelines on how to design the structural defects towards high performances are still missing. We use pulsed laser deposition (PLD) to carry out model experiments where we tune the microstructure and the surface morphology of the LaFeO3-δ oxide thin film in resistive switching devices. We show successful fabrication of epitaxial, polycrystalline and amorphous LaFeO3-δ films on single crystalline substrates. Stable hysteretic current-voltage profiles indicating memristance were measured for over 1000 consecutive cycles on the amorphous and polycrystalline thin films. Epitaxial, defect-free, films do not show reproducible nor stable resistive switching. Electrochemical Impedance Spectroscopy allows us to separate the dielectric response of the switching oxide from the electrodes and allocate the switching in the LaFeO3-δ layer. The high defect density in the amorphous films leads to an increase of the band-gap and activation energies by 0.2 eV and 0.13 eV, respectively. Furthermore a change in the near-order structure of the switching oxide LaFeO3-δ is apparent by Raman spectroscopy. In conclusion, we report for the very first time the operation of a new oxygen anionic-electronic resistive switching material, LaFeO3-δ, and also demonstrate the importance of high defect concentrations for reproducible resistive switching by a designed model experiment which is crucial for resistive switching optimization.

Resume : The diffusion of ions in dense ceramics of the low-temperature phase of tantalum pentoxide, L-Ta2O5, was examined by means of diffusion experiments and subsequent analysis of diffusion profiles with Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). 18O/16O isotope anneals were used to investigate oxygen diffusion, and oxygen tracer diffusion coefficients were obtained for the temperature range of 623 <= T/ K <= 873 at an oxygen partial pressure of pO2 = 0.2 bar. Cation diffusion in Ta2O5 was probed by using chemically similar niobium as the diffusant (in the absence of stable tantalum isotopes). Thin films of Nb2O5 were deposited onto Ta2O5 ceramics; niobium diffusion coefficients were determined for the temperature range of 1073 <= T/ K <= 1223 at an oxygen partial pressure of pO2 = 0.2 bar. Comparison of the measured diffusion coefficients indicates that oxygen is many orders of magnitude more mobile than niobium in L-Ta2O5 at these temperatures and under oxidising conditions. Conductivity data in the literature were transformed with the Nernst–Einstein equation into diffusion coefficients, which were also compared with measured diffusion data.

Resume : SrTiO3 (STO) and Gd2O3-CeO2 (GDC) are well-known model materials for mixed ionic electronic and ionic conductor, respectively, and were therefore chosen to get new insights into fundamental memristive switching mechanism. Thin films of both oxides were deposited on sapphire substrates by pulsed laser deposition method. The obtained films were characterized by X-ray diffraction, cyclic voltammetry and scanning electron microscopy. Two planar electrodes were deposited for impedance spectroscopy studies of the films and samples with typical dimensions of 1.5 x 3 mm2 were prepared. The coaxial line was used for the impedance measurements in the frequency range from 300 kHz to 3 GHz [1]. The analysis of the obtained electrical parameters was performed by applying distribution of relaxation times calculation methodology. STO and GDC thin films can be used to produce memristors [2, 3], whereas the current work aims to clarify the nature of change in impedance and electrical response for memristive switching on the given oxides. Acknowledgement. The research leading to these results has received funding from Lithuanian-Swiss cooperation programme to reduce economic and social disparities within the enlarged European Union under project agreement n° CH-3-ŠMM-02/06. [1] A. Kezionis, et al., IEEE Trans. Microw. Theory Tech., 62(10) 2456–2461 (2014). [2] F. Messerschmitt, et al. Adv Funct Mater 25, 32, 5117 (2015). [3] F. Messerschmitt, et al., Adv Funct Mater 24, 47, 7448 (2014).

Resume : The electrical conductivity and the crystalline structure of zirconia-10 mol% scandia-x mol% niobia solid electrolytes were investigated for x=0.25, 0.50 and 1.00 mol%. Dense specimens with relative densities higher than 95% were obtained by solid state reaction and sintering at 1500ºC for 5 h. Full stabilization of the cubic structure at room temperature was obtained for compositions with x=0.50 and 1.00, whereas for x=0.25 cubic and rhombohedric structures coexist. The electrical conductivity determined by impedance spectroscopy is of the same order of magnitude as that of the parent solid electrolyte (zirconia-10 mol% scandia) in the high temperature range (above 600ºC). The isothermal conductivity of the solid electrolyte with x=0.50 mol% niobium oxide remains constant up to 100 h at 600ºC.

Resume : Resistive random access memory (RRAM) is a non-volatile switching device for data storage that has great potential due to the long-time data retention, low operation voltage, and fast switching speed. RRAM based on electrochemical metallization has multilayered structure consisting of active metal/insulator/inert metal. According to electric potentials, conductive filaments are constructed and broken repeatedly in the insulating medium. However, conventional RRAM has shown a large fluctuation in the switching behaviors due to the random formation of the filaments, which deteriorates the reliability and durability of the devices. Herein, we have demonstrated an effective approach to fabricate RRAM devices with reliable switching behaviors via geometrical structuring of metal electrodes. Ag pyramid arrays are prepared and employed as an active electrode. When applying an electric potential to the electrode, the electric fields are largely enhanced at the tip of the pyramids. This leads to the facile development of filaments in the insulating medium at low electric potentials. Also, since the filaments are formed in a confined region, that is the tip of the pyramids, the resulting RRAM exhibits uniform operating voltages. Moreover, such low and uniform operating voltages result in the high improvement in the durability of RRAM. The RRAM devices obtained from structured metals will therefore provide excellent performance for next-generation memory.

Resume : Due to their impressive characteristics such as fast switching performances, high retention, cycling endurance and scalability, memristors are considered as a promising solution for memory and data storage. The memristor, introduced theoretically by Chua [1] and presented experimentally by HPLabs [2] is a two terminal device where resistance is modulated by application of an electric field. Here we present an oxide based memristor consisting of TiO2-x/TiO2 bilayer structure between two Pt electrodes. In this particular devices, resistance is modulated by controlling the Vo2 distribution in the oxide stack. The switching mechanism in our devices is identified to be bipolar and interfacial [3]. Under an applied external electric field, Vo2 are attracted to or repealed from the TiO2/Pt interface modifying the concentration of defects near the electrode and thus the Schottky barrier. By varying the thickness of TiO2-x layer, different mechanisms and thus different electrical characteristics can be obtained. Huge modeling efforts have been done lately [4] but a deep insight is still needed to completely understand the physical origin of the switching in these systems. Toward this goal, we propose a Fourier type analysis of our experimental results allowing us to extract some pertinent characteristics important for understanding the physics of memristors and to distinguish between the different types of observed switching. [1] L. O Chua, IEEE Transactions On Circuit Theory, Vol. CT-18, No. 5, September 1971. [2] D. B. Strukov, Nature, Vol 453, 1 May 2008. [3] G Sassine et al., JVSTB 2016 in press. [4] E. Linn, IEEE Transactions on Circuits and Systems—I: Regular Papers, Vol. 61, No. 8, August 2014.

Resume : The growth of oxide layers on metals is determined by formation and migration of point defects in the forming oxide layer and thus is an issue of fundamental importance. The main interest is to understand the processes that limit the oxide growth (e.g. surface reaction or chemical diffusion), and increase the reaction rate for possible data storage applications. In spite of their small thicknesses, the oxidation of Cr, Al, Ti, V, Zn, Ni and Co films (10 - 150 nm thick) is found to be controlled by diffusion through the oxide layer. The reasons for this are discussed. Ni films were doped with Cr as donor to increase the Ni vacancy concentration and thus nickel diffusion. However, the inhomogeneous Cr distribution in the growing oxide film prevented a faster oxidation. For comparison, ceramic Cr-doped NiO samples were studied. The obtained conductivities and chemical diffusion coefficients indicate a much lower solubility limit than reported in literature. The observed increase of the chemical diffusion coefficient at 700 °C is larger than one order of magnitude.

Resume : The substitution of lanthanum or molybdenum by other elements in the fast oxide-ion conductor La2Mo2O9 (LMO) leads to the so-called LAMOX family. This type of materials could be used as solid electrolytes in solid oxide fuel cells. There are few works dedicated to research of charge relaxation phenomena in LAMOX family compounds. Most solid ion conductors exhibit non-Debye type relaxation behavior, which rather would be represented by a continuous distribution of relaxation times (DRT). In many cases DRT representation may provide more information about processes on microscopic level than widespread representation by equivalent circuits. The accurate broad-band measurements of impedance up to frequencies of several GHz and high temperatures are required to obtain reliable DRT. We present a detailed study of electrical properties of La to Y substituted LMO (La1.8Y0.2Mo2O9 – LYMO) ceramic sample in wide temperature range, which covers a LMO inherent phase transition region (PT). DRT function for mobile ions was calculated by Tikhonov regularization method. The fine structure of the DRT function in the vicinity and far away from PT is investigated and compared for LMO and LYMO. The research leading to these results has received funding from Lithuanian-Swiss cooperation programme to reduce economic and social disparities within the enlarged European Union under project agreement n° CH-3-ŠMM-02/06.

Resume : The knowledge regarding distribution of dopants and oxygen vacancies in solid oxide electrolytes is crucial for understanding of structure-property relation (i.e. sample preparation & defect distribution - ionic conductivity). In this work we describe the configurational energy in terms of effective cluster interactions obtained from density functional theory calculations, which in its turn are used in Monte Carlo simulations to study the spatial distribution of dopants and oxygen vacancies over the wide range of Gd concentrations and temperatures.

Resume : Solid-oxide fuel-cells (SOFCs) are an attractive technology for efficient and clean energy conversion which is applicable to a wide variety of fuels (natural gas, liquefied petroleum gas, methanol and hydrogen)[1]. Current generation SOFCs require high operating temperatures in the region of 1000°C, this is primarily due to the high temperatures required by the electrolyte (usually yttria-stabilized zirconia) in order for oxide ion diffusion to occur. This high operating temperature contributes greatly to the cost and speed of degradation of these devices. LaGaO3 doped with Sr(II) on the A-site and Mg(II) on the B-site has been suggested as an alternative electrolyte material which can operate in the intermediate temperature range of 600-800°C.[2] Here we present the derivation of a doped-LaGaO3 interatomic potential, where the dopants are Sr(II) on the A-site and Mg(II) on the B-site. This interatomic potential is a dipole-polarizable ion model (DIPPIM)[3], which is fitted to force, stress and dipole data obtained from DFT calculations. When fitting to ab initio data as opposed to experimental data details on the potential energy surface which are non-equilibrium can also be accounted for. This increases the number of observables which can be reproduced. [1] Boudghene et al, Renew. Sust. Energ. Rev., 6, 433-455 (2002) [2] Morales et al, J. Eur. Ceram. Soc., 36, 1-6 (2016) [3] Castiglione et al, J. Phys: Condens. Matter, 11, 9009-9024 (1999)

Resume : The conductive bridging RAM (CBRAM) is one of the resistive RAMs (ReRAMs) and has high potential as a nonvolatile memory. In recent years, investigation using in-situ TEM (transmission electron microscopy) has been performed to understand its operation in real space. The scope of many works was to understand electrochemistry in CBRAM layer, and they were not to understand switching operation in realistic CBRAM device. To achieve speedy and stable switching operation, CBRAMs composed of two insulator layers are widely investigated. In this work, we performed in-situ TEM observations to study a double-insulator CBRAM of MoOx/Al2O3 sandwiched between Cu and TiN electrodes. By applying positive voltage to the Cu electrode, conductive filament was formed in both MoOx and Al2O3 layers. Its diameter was much smaller in Al2O3 (5 nm or less) than that in MoOx. Reducing the switching current, contrast of the filament was not identified in TEM images while clear resistive switching was still achieved. It was thought very thin filament contributed to switching operation. This work was supported under KAKENHI by JSPS (Nos. 25420279, 26630141, 15H01706). The Mitsubishi Foundation, Nippon Sheet Glass Foundation for Mater. Sci. & Eng, and the Scholar Project of Toyota Phys. & Chem. Res. Inst. are also acknowledged. Experiments were partly performed under Nanotechnol. Platform Prog. (Hokkaido Univ. and Kyushu Univ.) and Coop. Res. Prog. of “Network Joint Research Center for Materials and Devices” (Osaka Univ.).

Resume : The repassivation mechanism [1] of Al shows a temporal change from the bare metal to the fully-grown passive layer, which can be described by a transition from a point defect model (PDM) [2] to a high-field model (HFM) [3] of oxide growth. The observed smooth model transition during the repassivation current-decay occurs independently of the active electrode size and effective repassivation time for all applied overpotentials. Furthermore, the ratio of the total passivation time to the model transition approaches a constant value after repetitive laser treatments, pointing towards a laser-induced defect-centre saturation in the heat-affected and shock-affected zones. Charge transport in the growing oxide film on Al induced by in-situ nanosecond-laser depassivation [4,5] of plasma electrolytically oxididized (PEO) coatings was recorded in real time and evaluated according to well established theories of oxide-film formation and growth kinetics. This provides insight into the non-linear transient behaviour of micro-defect passivation and gradual defect-centre annihilation from the blank metal to the fully developed insulating oxide film. [1] T. O. Nagy et al., Appl. Surf. Sci. 302 (2014) 184-188. [2] D. D. Macdonald, J. Electrochem. Soc. 139 (1992) 3434-3449. [3] T.P. Hoar et al., J. Phys. Chem. Solids 9 (1959) 97-99. [4] A. Cortona and W. Kautek, Phys: Chem. Chem. Phys. 3 (2001) 5283-5289. [5] W. Kautek and G. Daminelli, Electrochim. Acta 48 (2002) 3249-3255.

Resume : Electric double layer (EDL)-gated carrier injection has attracted rapidly growing attention due to its capability to modulate material properties in the vicinity of the EDL interface. In most of such studies, ionic liquids have been used as an electrolyte to form EDL. In order to make this newly emerging technology into real devices, it is inevitable to realize all-solid-state devices working at room temperature. Here, we have demonstrated conductivity modulation of gold thin film by all-solid-state EDL-gating at room temperature at the speed of ~ 1 sec. The gold conducting channel was formed on top of an epitaxially grown bi-layer of (LaLi)TiO3 lithium ion conductor / SrRuO3 bottom electrode on an SrTiO3(100) insulating substrate. The gold channel conductivity was modulated by applying gate bias across the LLTO electrolyte layer. The sheet conductivity varies by 7.5e-4 (ohm/sq)^-1 which corresponds to the carrier density modulation roughly by order of ~ 10^14 /cm2. The time constant for the conductivity modulation was as fast as ~ 1 sec even at room temperature, which is several orders of magnitude faster than the previously reported solid-state EDL-gated transistor at 100 – 250 degree centigrade. The time constant of the conductivity modulation exponentially reduces with increasing applied gate bias, which presumably reflects upon the thermal activation process of ionic transport in the electrolyte.

Resume : In our previous work we have reported non-filamentary resistive switching based on bulk transport of oxygen ions and electrons in amorphous, highly non-stoichiometric gallium oxide thin films [1]. Modeling of the switching kinetics by electric field-driven oxygen migration and resultant local changes in electron conductivity indicated a link between the observed properties and the disordered nature of the films. In order to investigate the relationship between structure, stoichiometry and switching behavior, we prepared amorphous gallium oxide thin films with varied stoichiometry by Pulsed Laser Deposition (PLD) under reducing conditions. Oxygen tracer diffusion coefficients obtained with 16O/18O isotope exchange experiments and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were used to elucidate the role of oxygen diffusion in the resistive switching mechanism of gallium oxide. Electrical conductivity measured as a function of temperature by the van der Pauw method and resistive switching experiments showed that amorphous structure and sub-stoichiometry is beneficial in terms of total conductivity and switching properties. [1] Y. Aoki, C. Wiemann, V. Feyer, H. S. Kim, C. M. Schneider, H. I. Yoo, M. Martin, Nat. Commun., 5:3473 (2014).

Resume : La0.5Sr0.5Co0.5Mn0.5O3-δ (LSMC) is one of the few reported perovskite oxides that is stable with δ > 0.5, accommodating a fully oxidised phase (δ = 0) and a reduced phase (δ = 0.62) [1]. The reversible, topotactic transition between these two phases means LSMC is an ideal material for the investigation of defect structure tuning in perovskites. Understanding defect structure is critical for developing oxygen reduction and evolution mediators. Thin films of LSMC (15-40 nm) were deposited using pulsed laser deposition on single crystal substrates with compressive strain on LaAlO3 (LAO) and tensile strain on SrTiO3 (STO) with lattice mismatches of -1.50% and 0.46% respectively. Initial in situ X-ray diffraction studies were conducted at elevated temperatures (400 – 750°C) and varying oxygen pressure [2]. The results demonstrate that thin film LSMC responds to changes in the oxygen partial pressure, with oxidation processes occurring within minutes. Films grown on LAO showed a five times greater cell parameter change between oxidising and reducing conditions compared to those grown on STO, with changes of 3.21x10-3 Å and 6.6x10-4 Å at 700°C respectively. These results suggest that the initial constraints placed on the LSMC by the strained growth restrict the possible oxidation states of the transition metals and provide evidence for the possibility of tuning redox process taking place in perovskites by strain. These findings are of importance for improving catalytic performance of perovskites under different in situ conditions for a range of applications including fuel cells, electrolysers and oxygen store devices. [1] Aguadero, A. et al. Angew. Chem. Int. Ed. Engl. 50, 6557–61 (2011). [2] Moreno, R. et al. Chem. Mater. 25, 3640–3647 (2013).

Resume : TiO2 thin films have drawn a lot of attention for their application in emerging memory devices, such as resistive random access memory (ReRAM). However, TiO2 ReRAM still faces reliability issues, including poor endurance, large device-to-device and cycle-to-cycle variability of switching parameters and low yields. Moreover, high electroforming voltages have been often associated with irreversible damage to devices. Doping of TiO2 has been employed as a strategy for overcoming these issues. Therefore in this work, we used Al as a dopant in TiO2 thin films to investigate its effect on electroforming and switching voltages of ReRAM devices. Conductive atomic force microscopy (C-AFM) measurements on these thin films, suggested that Al doping decreased the switching voltages compared to the undoped thin films. This result was confirmed by pulse voltage sweeping of ReRAM devices employing the same doped thin films. The Al-doped devices were on average electroforming at -5.7 V, compared to -6.4 V for the undoped ones, and they were switching with potentials as low as ±0.9 V. These findings suggest a potential pathway for implementing low-power ReRAM systems.

Resume : Conductive Bridge Random Access Memories (CBRAM) is one of the most promising non-volatile memory technologies due to their properties such as high scalability, low power consumption and high switching speed. CBRAM rely on the formation/dissolution of a metallic-rich (Ag or Cu) conductive filament within a solid-electrolyte. Germanium Sulfide (GeSx) is a very interesting solid electrolyte material studied so far. To achieve resistance switching, GeSx has to feature sufficient sulfur content (>50%) in order to facilitate cation migration. However sulfur has a very high vapor pressure (e.g. sulfur evaporates at 19°C at 1e-6 Torr) which makes deposition of GeSx (x>0.5) by sputtering under inert atmosphere challenging. In this study, we report on the fabrication of Ag\GeSx\W CBRAM devices obtained from PVD of a GeS target. EDX analyses show that sulfur content of the samples gradually decreases over time which in turns affects the devices functionnality dramatically. We show that inserting an additional Ag layer between the W bottom electrode and the GeS layer restores functionnal devices. XRD measurements reveal the formation of an Ag2GeS3 phase through the sulfurization of the inserted Ag layer which acted as a sulfur gettering layer. Following this procedure CBRAM devices exhibit interesting bipolar behavior where SET can be achieved in both polarity. These results are analyzed and explained by means of a dedicated physical model involving ionic motion heat equation.

Resume : SrSiO3 and 40mol% Na-doped SrSiO3 have been synthesized using solid state reaction method. The as-sintered materials are structurally characterized using XRD and Raman analysis. The XRD results shows that single polycrystalline phase is formed in the present system. Na-content slightly distorts the parent phase, which may leads to modification in the monoclinic structure. The Raman analysis indicates some glassy phase co-formed in 40mol% Na-doped SrSiO3 system. The conductivity behavior of the samples were investigated using ac impedance spectroscopy. The electrical conductivity of Na-doped system is 22.8 mS/cm at 800 C in air.

Resume : With conventional flash based non-volatile memories (NVM) gradually approaching its limitations of scaling, resistive random access memory (RRAM) based on CMOS compatible metal oxides (HfOx, TaOx etc.) is among the widely investigated choices of next generation NVM. Techniques like oxygen engineering [1] and doping is of interest for higher device yields and improving the device performance by reduced forming voltages in TaOx based RRAM devices. The optical band gap of the amorphous TaOx films grown by varying the oxygen radical flow in MBE was found to be tunable in a wide range (3.6 to 4.6 eV). Density and surface oxidation in ambient atmosphere of these Ta2O5/TaOx film stacks were further characterized using X-ray reflectivity (XRR). Quasistatic DC switching measurements performed on Pt/Ta2O5/TaOx/TiN devices (30x30 m2) indicated stable bipolar resistive swithcing in all devices. Forming voltages of these devices showed a strong dependence on the oxidation conditions indicating high tunability from 1.5 V to 10 V for 20 nm oxide thickness, thereby suppressing its thickness dependence. [1] S. U. Sharath et al., Appl. Phys. Lett. 104, 063502 (2014).

Resume : Resistive switches are extensively studied as non-volatile memories, being promising building blocks of future electronics. Despite the known importance of oxygen vacancies in metal-oxide memristive devices, no systematic studies have been conducted on well-defined systems. In this work, we utilized Ce1-xGdxO2-y to tune the oxygen vacancy concentration in order to correlate the structural and memristive properties. We fabricated a series of memristive cross-point devices based on ceria films and Pt electrodes with Gd content varying from 3 to 30 mol%, thus systematically changing the oxygen vacancy migration and defect association energies. Based on hysteretic current-voltage profiles, we found a maximum in resistive switching (with ROFF/RON of 100) for 20 mol% Gd, at which the maximum ionic conduction of the material was also observed. In contrast, for low and high Gd-concentration the oxides reveal no resistive switching, since there are not sufficient mobile ionic carriers. The oxygen vacancy content is either too low (low Gd %) or the mobility is reduced due to formation of defect clusters (high Gd %) which is visible by the appearance of new vibrational modes in Raman spectroscopy due to the reduced symmetry of the system. The results were found to be independent of electrode area and film thickness. This report presents Ce1-xGdxO2-y as an excellent model material to systematically study variation of ionic conduction in memristive devices. Clear correlation between concentration and mobility of oxygen ionic carriers and resistive switching characteristics are found. Implications of this work include fundamental and direct insights on the requirements of oxygen vacancy mobility for nonvolatile memory and logic applications.

Resume : Traditionally the working principle of electrochemical metallization resistive switching devices has been explained in terms of metallic filament formation. Contact metal ions dissolve into an insulating oxide, drift toward the counter electrode and start forming a filament that eventually would connect the two metal contacts turning the device from a high resistance state (HRS) to a low resistance state (LRS). However, some studies on a ZnO nanowire device showed that LRS might be achieved even without formation of a continuous filament. This rises up questions about the applicability of the conventional theory to nanowire-based devices. By means of Density Functional Theory simulations, we propose an alternative switching mechanism that involves metal adatoms migrating at the NW surface. Contact atoms rather than forming a filament, spread on the surface. Adatoms, which are extracted from the electrode, are the species that most of all contribute to conduction. Density of states calculations show that adatoms locally dope the surface of the NW creating a surface conductive channel on the NW external shell. Adatom distribution (and therefore the resistance state of the device) is tuned by the external voltage that easily move them back and forth once the electric field intensity exceeds the diffusion barrier. This model predicts switching occurrence even for low densities of contact ions in accordance with the large dimensions of the NW employed in the experimental device.

Resume : In this work we studied the mechanisms behind defect generation in amorphous SiO2 (a-SiO2) under electron injection conditions using density functional theory (DFT). The application of electric fields across thin layers of amorphous oxides is widely believed to create defects and change the physical properties of these materials. This principle is currently of great interest in metal-insulator-metal resistive RAM (ReRAM) devices. Due to the varying local geometry in amorphous materials, some intrinsic sites composed of wide O-Si-O bond angles can act as deep electron traps. Our calculations show that these traps can accommodate up to two electrons which further increases the O-Si-O bond angle to nearly 180 degrees and stretches the corresponding Si-O bonds to the breaking point. These weakened bonds result in much lower activation energies and an efficient bond breaking pathway for producing O vacancies and interstitials. Newly generated O interstitials are shown to migrate rapidly through the oxide to the positive electrode while O vacancies remain behind. Furthermore, we show that this formation mechanism is only weakly correlated when the concentration of existing vacancies in the system is low. Combining these findings we have developed a mechanism for electron injection facilitated defect formation in a-SiO2. These results are discussed in the context of experimental data on time dependent dielectric breakdown, memory devices, and radiation damage.

Resume : Nanoionics has become an increasingly promising field for the future development of advanced energy conversion and storage devices, such as batteries,fuel cells, and supercapacitors. Particularly, nanostructured materials offer unique properties or combinations of properties as electrodes and electrolytes in a range of energy devices. However, the enhancement of the mass transport properties at the nanoscale has often been found to be difficult to implement in nanostructures. In this talk, we will detail the fabrication of an artificial mixed ionic electronic conducting oxide obtained by grain boundary (GB) engineering thin films of La0.8 Sr0.2 Mn O3+δ . This electronic conductor is converted into a good mixed ionic electronic conductor by synthesizing a nanostructure with high density of vertically aligned GBs with high concentration of strain-induced defects. Since this type of GBs present a remarkable enhancement of their oxide ion mass transport properties (of up to six orders of magnitude at 773 K), it is possible to tailor the electrical nature of the whole material by nanoengineering, especially at low temperatures. Complementary, we will analyse the effect of the GB contribution to the mass transport properties of other interface-dominated thin films that show initial mixed ionic electronic conduction, e.g. La0.8 Sr0.2 Co O3-δ . A comprehensive work of the LSMC family fabricated with combinatorial Pulsed Laser Deposition (La0.8 Sr0.2 Mnx Co1-x O3-δ , x=0-0.8) will be discussed. The results of this study leads to fundamental insights into oxygen diffusion along GBs and to the application of these engineered nanomaterials in new advanced solid state ionics devices such are micro-solid oxide fuel cells or resistive switching memories.

Resume : The highest conductivity reported in any oxygen ion conducting solid electrolyte is the high temperature fluorite-structured delta-phase of Bi2O3, that is only stable in a narrow temperature range of 705°C to 825°C [1]. The ionic conductivity is associated with the high intrinsic oxygen vacancy concentration of 25%, well above the typical values of vacancy concentrations ~5-10% achieved through aliovalent doping strategies in other fluorite-structured systems such as Gd-doped CeO2 (GDC) and yttria-stabilised ZrO2 (YSZ). The low temperature, monoclinic, alpha-phase of Bi2O3 has an oxygen-ion conductivity 7 orders of magnitude lower than the delta phase, which is as expected in a low symmetry structure, with no inherent vacancies to facilitate ion migration. The study by Bayliss et al [1] using oxygen-18 isotopic tracer and SIMS depth profiling methods to quantify the mass transport properties of the alpha- and delta-phases of Bi2O3 showed that the measured oxygen tracer diffusion profiles in the low temperature alpha-phase of Bi2O3 always deviated from a single bulk diffusion (D*bulk) process. The origin of these secondary, fast diffusion paths was assigned to fast grain boundary diffusion (D*gb) with the proviso that the fast conduction path could possibly be due to materials defects, such as porosity or cracking allowing gas phase permeation, or equipment artifacts present, such as SIMS sputter variations and crater roughening. The values for grain boundary oxygen diffusion (D*gb) in alpha-phase Bi2O3 were reported as several orders of magnitude below those for bulk delta-phase, nevertheless they are comparable to the diffusivities measured for bulk CGO. More recently, work on the perovskite oxide La0.8Sr0.2MnO3+delta by Saranya et al [2] and Navickas et al [3], demonstrated that through microstructural engineering of nanostructures with a high density of aligned grain boundary and a high concentration of strain-induced defects that LSM can be transformed from a “bulk” electronic conductor into a “bulk” mixed ionic and electronic conductor through the same concept of grain boundary utilization. Through these two examples it has been demonstrated that both electrolyte-type and electrode-type materials can be engineered through purely microstructural control utilizing the enhanced mass transport properties of the grain boundary regions. Furthermore, grain boundaries present at the surface are also at the gas solid electrolyte interface where they may offer enhanced exchange kinetics, this is important when we consider the oxygen reduction reaction remains a large fraction of the total area specific resistance in current day solid oxide fuel cells. [4] The results reported in this contribution are an extension of the work presented in Bayliss et al [1] in which the secondary oxygen conduction path in the alpha-phase of bulk Bi2O3 is explored using oxygen-18 tracer exchange at 600°C and analysed by SIMS and TEM. The SIMS instrument used in this study is an ion microscope with nanoscale resolution and imaging capabilities for distinguishing extended defects in the bulk Bi2O3 material such as pores and cracks. The issues associated with the SIMS sputter-etching technique have also been addressed using optical interferometry techniques. Sections of a sample containing fast oxygen conduction paths identified by oxygen-18 tracer exchange and SIMS mapping have been prepared as thin, cross-section foils for TEM examination allowing atomic level structural investigations. Pores and cracks are identifiably different to grain-boundaries in this combination of analytical techniques, confirming that, in the Bi2O3 alpha-phase, grain boundary oxygen ion transport is fundamentally different from that in the bulk. 1. Bayliss, R.D., et al., ADVANCED ENERGY MATERIALS, 2014. 4(10). 2. Saranya, A.M., et al., ADVANCED ENERGY MATERIALS, 2015. 5(11): p. 6. 3. Navickas, E., et al., PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 2015. 17(12): p. 10.

Resume : Donor doped barium titanate is of high technological relevance in view of its wide-spread applications, such as PTC (positive temperature coefficient) resistors. Moreover, n-type BaTiO3 ceramics represent an important model system for interfacially controlled electroceramics. It is the aim of this contribution to present conductivity relaxation experiments on n-type barium titanate ceramics, co-doped with Y (donor) and Mn (acceptor), as a function of temperature (700 – 900°C) and oxygen partial pressure (0.001 – 0.5 bar) by employing the van der Pauw technique. The chemical diffusion process owing to a change of the oxygen partial pressure in the surrounding atmosphere can be described by fast transport of oxygen vacancies along the grain boundaries (mainly core regions) and slow diffusion of cation vacancies into adjacent bulk regions (including space charge layers). A careful analysis of the experimental conductivity relaxation experiments allows the determination of the chemical diffusion coefficient for oxygen along grain boundary core regions with typical values in the range of 10-8 to 10-9 cm2 s-1. Furthermore, the partial pressure dependence of the electronic conductivity can be interpreted by means of a defect chemical model involving grain boundary core regions where Mn is segregated. The significant interaction of the segregated acceptor dopants with electrons (trapping) plays a decisive role for the evolution of the interfacial charge density and Schottky barriers.

Resume : In the past decade solid oxide fuel cell (SOFC) research mainly focused on the cathode side and vast improvement was achieved by substituting the triple phase boundary active lanthanum manganite (LSM) electrodes with mixed ionic electronic conductors (MIEC) such as lanthanum cobaltite based electrodes (LSC and LSCF). SOFC anodes, however, still rely on nickel/yttria-stabilized zirconia (YSZ) cermet structures. An analogous development to MIEC materials on the anode side is therefore expected to significantly improve SOFC performance. One promising candidate is gadolinia doped ceria (GDC), which has been reported in literature to exhibit high H2S tolerance as well. In this contribution thin film GDC anodes with specially shaped buried current collectors were applied, which allow a separation of elementary processes such as electronic and ionic conductivity, surface exchange resistance and chemical capacitance. By means of these model composite anodes it is possible to show that GDC anodes undergo significant electrochemical changes in H2S containing atmospheres. Moreover, the sulfur poisoning of GDC anodes can be mainly attributed to an increase of the surface exchange resistance, while the other probed parameters undergo only minor changes relevant to SOFC performance. In addition sulfide uptake into GDC bulk was investigated by secondary ion mass spectrometry (SIMS) and diffusion as well as surface exchange coefficients were estimated from the obtained diffusion profiles.

Resume : Mixed ionic-electronic conducting perovskite oxides (ABO3) are used for ceramic high-temperature (700…900 °C) oxygen-transport membranes (OTMs). Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) shows outstanding oxygen-permeation properties and is the most promising candidate for such an OTM. Its superior performance, however, depends on its cubic perovskite structure. This presents a problem at temperatures below 840 °C where detrimental secondary-phase formation occurs – preferably along the grain boundaries. By suitable B-site doping with, e.g., yttrium, the cubic phase stability range can be expanded to lower temperatures. Generating an atomic three-dimensional (3D) reconstruction of the chemistry of perovskite ceramic bulk samples by means of atom probe tomography (APT) enables determination of the exact phase composition in the vicinity of the grain boundaries. To this end, ceramic BSCF and BSCF10Y (Ba0.5Sr0.5(Co0.8Fe0.2)0.9Y0.1O3-δ) bulk samples were prepared and annealed at 800 °C for 10 d in ambient air. To analyze the element distribution in the grain boundaries by APT, tips with diameters between 50 and 200 nm were cut out by the focused ion beam (FIB) technique. APT was used to analyze the nanoscale chemistry of BSCF and BSCF10Y before and after aging with respect to a chemical element analysis of the grain boundary regions. By comparing the doped and undoped BSCF the influence of yttrium on the formation of the hexagonal phase can be determined.

Resume : BaZrO3 thin films have attracted considerable attention for potential applications in electrochemical devices such as solid oxides fuel cells, sensors and hydrogen pumps, due to high proton conductivity and good chemical stability. In this study, we performed first principles calculations combining the linear combination of atomic orbitals approach with the hybrid functional for investigation of the structural, electronic and vibrational properties of neutral and fully charged oxygen vacancies in ultrathin perovskite films. The non-stoichiometric, symmetrical (001)-terminated 2D slabs (both BaO and ZrO2 terminations), with a number of atomic planes ranging from 3 to 7 were mostly considered; corresponding to a free standing film of up to ca. 2 nm thickness. Special attention was paid to the analysis of the vibrational contribution to the Gibbs formation energy, ΔGvib, of vacancies in slabs of different thickness and terminations. It is significantly larger for charged than neutral vacancy, both in bulk and in all films, due to larger lattice distortion and relevant blue shift of the phonon spectrum. Thus, for charged vacancy at 1000 K, ΔGvib~1 eV for all slab thicknesses, while it is almost negligible for neutral vacancy, except for 3-plane films, where the lattice distortion leads to a blue shift for some phonons and ΔGv ~0.3 eV. Calculations of the neutral vacancy formation enthalpy show that it is close to the bulk value (~7 eV), also except for such thinnest 3-plane films, where it is reduced by ~0.5 eV. Thus, the Gibbs formation energy even for 3 plane slabs is close to the bulk one and the confinement effects for oxygen vacancies (color F centers) in BaZrO3 thin films (and probably in other analogous ionic-covalent ABO3 perovskites) are very short-range.