Functional perovskite systems

Resume : Ferroelectric tunnel junctions (FTJs) have opened up promising routes towards energy-efficient data storage applications and memristive devices. Polarization reversal in a ferroelectric tunnel barrier can change the resistance of a junction, an effect known as tunneling electroresistance (TER). Large TER has been obtained for junctions that are comprised of different electrodes. Here, we demonstrate that a giant TER effect of 10 000 000% can be realized in nominally symmetric junctions of two La2/3Sr1/3MnO3 (LSMO) electrodes separated by a PbZr0.2Ti0.8O3 (PZT) or BaTiO3 (BTO) tunnel barrier. The TER is ascribed to structural dissimilarities at the barrier/electrode interfaces and their influence on a polarization-induced metal-to-insulator transition in LSMO. Evidence for this scenario is obtained from transmission electron microscopy and in-plane electric transport measurements inside a piezo-response force microscope. From these data, it is estimated that a 0.8 nm thick LSMO interface layer becomes insulating when the polarization of the ferroelectric barrier points towards the bottom electrode. This effect increases the tunnel barrier width and it enhances the tunnel barrier resistance. A similar effect does not occur when the polarization points to the LSMO top electrode due to roughness and atomic mixing at the interface. The robust TER response indicates that interface engineering of FTJs provides a viable scheme for the design of sensitive resistive switching devices.

Resume : Transition metal oxides with a perovskite crystal lattice of type ABO3 may possess corresponding oxygen-deficient modulation structures with different material properties. One prototypical example is the brownmillerite structure of type ABO2.5, which due to its high ionic conductivity could find applications in solid oxide fuel cells, oxygen-separation membranes, and gas sensors. Here, we demonstrate that the evolution of the perovskite-brownmillerite phase transition can be fully controlled and monitored in epitaxial La2/3Sr1/3MnO3 (LSMO) films using electron-beam irradiation in a transmission electron microscope (TEM). Real-time TEM imaging with atomic resolution reveals that the structural transition is driven by an incessant ordering of electron-beam induced oxygen vacancies in every second MnOx plane. The local depletion of oxygen reduces the coordination of Mn cations, causing a vertical displacement of the La/Sr ions. Over-irradiation of the brownmillerite phase induces a second transition to a perovskite-like structure with disordered oxygen vacancies and an enhanced out-of-plane lattice compared to the original LSMO film. Electron energy loss spectroscopy and energy-dispersive x-ray spectroscopy confirm the findings. The demonstrated ability to simultaneously induce and characterize oxygen-deficient structural phases in a continuous and controllable manner opens up new pathways for atomic-scale studies on electronic and ionic transport in complex oxide materials.

Resume : The two dimensional electron gas at the (001) interface between LaAlO3 (LAO) and SrTiO3 (STO) is generally explained considering a scenario of polar catastrophe. Remarkably, (110)- and (111)-oriented crystalline LAO/STO interfaces as well as the interface between STO(001) and some amorphous oxides can be also conducting. For the amorphous/STO interfaces, oxygen vacancies are usually claimed as the origin of the conductance. Redox reactions may depend on two factors: the oxygen affinity of the ions in the amorphous layer and the vacancy formation energy and diffusivity. Generally, the latter depends on the crystal orientation. To evaluate the relative weight of both factors, we have grown series of amorphous -LAO, YSZ and STO films on STO(110) and STO(111). The critical thickness for conduction depends on the amorphous oxide. However, it is the same for LAO/STO(110) and LAO/STO(111), strongly suggesting that differences vacancy formation energy and diffusivity are overridden by interface redox processes, which are governed by the oxygen affinity of the metal ions in the amorphous layer. TEM revealed a crystalline interfacial layer around 0.5 nm thick and EELS signaled presence of oxygen vacancies. In conclusion, the oxygen affinity of the incoming ions, rather than the crystalline plane of the substrate, is the driving force of the interface redox reactions that triggers conduction at amorphous/STO interfaces.

Resume : Results regarding the pyroelectric and photovoltaic properties of the ferroelectric thin films will be presented. The main parts will be: - growth of good quality epitaxial ferroelectric films and preliminary structural/electrical characterization - short-circuit photocurrent measurements; dependence on the properties of the electrode interface; relation with the magnitude of the imprint - pyroelectric properties of epitaxial ferroelectrics - combined pyroelectric-photovoltaic properties; enhanced pyroelectric signal by continuous UV illumination - pyroelectric properties in multilayers

Resume : In the recent past, the field of photo-ferroelectrics has been revitalized by the reports of photovoltaic effect (PVE) in BiFeO3 (BFO). Unlike traditional semiconductors the open circuit voltages in these materials are not limited by band gap Eg, they can exceed Eg by orders of magnitude. The microscopic origins of this effect are still under debate. Initial investigations on BFO films assumed that the PVE in BFO is primarily due to the presence of a potential step at the domain wall. However, in our recent work it was observed that the generation and recombination of photo-generated non-equilibrium carriers are primarily affected by the presence of shallow energy levels and the anomalous bulk photovoltaic effect play the major role in PVE. This talk will address fundamentals of photoferroelectrics and anomalous bulk photovoltaic effect in BiFeO3. Besides of that, newly characterization methods based on local photoelectric measurement such as photo-induced transient spectroscopy (PITS) which bring valuable data regarding generation and recombination of the photo-excited carriers will be tackled. We will also discuss complementary characterization methods such as thermally stimulated current and variable temperature PV effect and show that these can provide information on electronic structure of material, respectively shallow or deep levels in the band gap, which may affect the dark and photo-conduction mechanism.

Resume : The study of the generation of GHz-THz coherent acoustic phonons by femtosecond laser action is a powerful time-domain spectroscopy to evaluate and possibly to control the electron-phonon coupling mechanisms in solids [1]. These fundamental investigations are a key issue for future development of ultrasonic devices providing short acoustic wavelengths (nm) required for advanced nanometrology. Up to now, photo-induced GHz-THz coherent acoustic phonons have been mainly explored in metals and semiconductors as well as in artificial nanostructures [1]. However, despite their inherent strong polarization (providing natural asymmetry) and superior piezoelectric properties, ferroelectric oxides have been regarded only recently [2-5]. Here by using ultrafast pump-probe optical measurements, we report that photo-generation/photo-detection of small band-gap BiFeO3 ferroelectric leads, at room temperature, to spectacular larger GHz coherent shear acoustic wave signal (TA mode) than that coming from longitudinal mode (LA mode), as never reported in any material before [6]. The detailed analysis of the data indicates that this singular behavior comes from an efficient light-induced inverse piezoelectric mechanism. In particular, this piezoelectric effect is governed by the screening of the internal electric fields in BFO driven by light-induced charges. This giant photoacoustic response opens new perspectives for the use of ferroelectric oxides in ultrahigh frequency acoustic devices and the development of new GHz-THz acoustic sources. [1] P. Ruello, V. Gusev, Ultrasonics, Review Paper, in press (2014). [2] P. Ruello, T. Pezeril, S. Avanesyan, G. Vaudel, V. Gusev, I. C. Infante, and B. Dkhil, Appl. Phys. Lett. 100, 212906 (2012) [3] L. Y. Chen, J. C. Yang, C. W. Luo, C. W. Laing, K. H. Wu, J.-Y. Lin, T. M. Uen, J. Y. Juang, Y. H. Chu, and T. Kobayashi, Appl. Phys. Lett. 101, 041902 (2012) [4] Z. Jin, Y. Xu, Z. Zhang, X. Lin, G. Ma, Z. Cheng, and X. Wang, Appl. Phys. Lett., 101, 242902 (2012) [5] H. Wen, P. Chen, M. P. Cosgri, D. A. Walko, J. H. Lee, C. Adamo, R. D. Schaller, J. F. Ihlefeld, E. M. Dufresne, D. G. Schlom, P. G. Evans, J. W. Freeland, and Y. Li, Phys. Rev. Lett. 110, 037601 (2013) [6] M. Lejman, G. Vaudel, I. C. Infante, P. Gemeiner, V. E. Gusev, B. Dkhil, and P. Ruello, Nature Comm. in press (2014)

Resume : In a material with both a piezoelectric effect and the capability to form a photocurrent, photostriction can be observed – the deformation of the crystal by light. When studying photostriction, a potential method is to use x-rays to probe the unit cell in response to another light source as a stimulus, such as a laser or a diode. Given that x-rays are also photons it is plausible that they produce some effect in themsleves. Experiments were carried out to investigate how significant are the effects of the x-rays are in producing photostriction in the absence of any other source of illumination. The material studied in this example was bismuth iron oxide, BiFeO3. A thin film with electrodes was used, and it was found that the photocurrent generated by a laboratory x-ray source on the sample was of comparable level to that of a laser or diode used in the study of the bulk photoelectric effect. Using a novel, time dependent crystallographic approach, the intrinsic effect of synchrotron x-ray light on a photoferroelectric thin film has been investigated. Furthermore, we have simultaneously collected diffraction and photoelectric data, and the correlation between the electronic and structural properties will be discussed. These results could suggest that caution is necessary when interpreting photostriction data obtained with the use of x-rays

Resume : To achieve innovations in photocatalysis new materials and material types are required. Barium titanate (BaTiO3) has a similar band gap and electron affinity to TiO2, suggesting the potential for similar photocatalytic behaviour, but its perovskite structure leads to additional ferroelectric properties. We study the photocatalytic activity of BaTiO3 via the degradation of a standard organic dye molecule, rhodamine B. We compare the impact of ferroelectricity on photocatalytic activity by making use of the stability of both cubic, non-ferroelectric and tetragonal, ferroelectric BaTiO3 at room temperature. We find that a catalyst with higher ferroelectric content displays three times higher photocatalytic activity than non-ferroelectric material despite a lower surface area. In addition, photochemically depositing silver nanoparticles on the surface of the ferroelectric catalyst leads to ten times higher activity than for the non-ferroelectric equivalent. We link this enhancement to the ability of the ferroelectric material to effectively separate photoinduced charge carriers due to the polarisation-induced internal field, and the strong Stern layer formed on the ferroelectric surface, which increases dye adsorption. The further enhancement of the silver-coated catalyst is linked to preferential deposition on the positive domains, where photo-reduction is more likely to occur. This highlights the potential of ferroelectric materials for use in photocatalytic applications.

Resume : The preparation of composite lead iodide perovskite/ZnO composite layers by impregnation and chemical conversion of a precursor soaking various electrochemically deposited ZnO structured films is presented. These layers have been used as photoelectrodes in excitonic solid-state solar cells with the structure: ZnO/perovskite CH3NH3PbI3/Spiro-OMeTAD. Well-conducting ZnO layers are deposited in chloride medium and grown with tailored (nano)structures ranging from arrays of nanowires to a compact, well-conducting film. Moreover, the effect of a thin intermediate overlayer of ZnO conformally electrodeposited in nitrate medium and with a low n-type doping (i-ZnO) is discussed. Our results show higher power conversion efficiencies for the nanostructured perovskite/oxide composite layers compared to the dense one. Moreover, we show that the presence of the i-ZnO layer in contact with the perovskite improves markedly the cell short circuit current and the open-circuit voltage due to charge recombination reduction. For the best cells, the active layers efficiently absorb light over a large spectral range from near-UV to near infra-red region and exhibit excellent charge collection efficiencies. Solar cells based on an optimized design generate a very large photocurrent and a power conversion efficiency above 10%.

Resume : KCa2NaNb4O13 ceramics, Dion-Jacobson layered perovskite materials, have been widely investigated because they can be easily exfoliated to the nano-sized sheets which can be used to the future multilayer capacitors with a small size and high performance. A KCa2NaNb4O13 phase was formed at 1000oC and a dense ceramics were obtained after sintering at 1325oC for 10 h. A proton exchange process was carried out to produce the intermediate phase of HCa2NaNb4O13. The stable (Ca2NaNb4O13)- nanosheet colloids were formed using the mixture of the HCa2NaNb4O13 powders, tetrabutylammonium hydroxide (TBAOH) solution and water having the same TBA+/H+ ratio. The exfoliated (Ca2NaNb4O13)- nanosheets were dispersed into the acetone medium, which was used to form the thin film by the electrophoretic method under 100 V electric source. The electrophoretic thin films were then annealed at various temperatures for 30 min under air condition and all the films have the Ca2NaNb4O13 phase. The electrical and dielectrical properties of these films will be presented in this work.

Resume : Homogeneous KTiNbO5 and K3Ti5NbO14 phases were formed at 900oC and the dense KTiNbO5 and K3Ti5NbO14 oxides were obtained from the specimen sintered at 1150oC and 1125oC, respectively. Liquid-phase-assisted abnormal grain growth occurred in both specimens along the directions perpendicular to <002> during the sintering. A proton exchange process was carried out to synthesize the proton-exchanged powders which reacted with tetrabutylammonium hydroxide (TBAOH) solution and DI water to synthesize stable [TiNbO5]- and [Ti5NbO14]- nanosheet colloids. These nanosheet colloids were dispersed into the acetone, which was used to grow the thin films by the electrophoresis under 100 V electric source. The TiNbO5 and Ti5NbO14 thin-films have the (001) preferred growth direction. A high dielectric constant(εr) of 48 with a loss of 4.0% at 100 kHz was obtained from the TiNbO5 film annealed at 600oC and this film also showed a low the leakage current density of 2*10-7 A/cm2 at 0.25 MV/cm. Moreover, the Ti5NbO14 films annealed at 700oC showed an εr of 41 with a dielectric loss of 1.5 % at 100 kHz and the leakage current density of 1.5*10-7 A/cm2 at 0.3 MV/cm. Therefore, these films are good candidates for the future multilayer ceramic capator.

Resume : Monolithic integration of functional oxides on Si facilitates the progress of several novel electronics and photonics devices. BaTiO3 (BTO), with its unique opto-electrical properties, is a promising candidate for such advanced applications. Yet, the integration of BTO onto Si is very challenging due to its different crystal structure and dissimilar lattice parameter. Moreover, BTO intrinsic properties are sensitive to crystalline phases defined by stoichiometry and epitaxial conditions.This work describes the co-deposition by molecular beam epitaxy of BTO single-crystal films on Ge-on-Si(001) (GoS) pseudosubstrates. To perfectly register both BTO and Ge lattices during heteroepitxy, an in-plane 45˚ rotation of the oxide lattice with respect to Ge(001) surface has been developed using a ½-monolayer BaO at the BTO/GoS interface. Then, the impact of stoichiometry on BTO properties is studied in terms of reflection high energy electron diffraction, x-ray diffraction, transmission electron microscope, Rutherford backscattering spectroscopy and ellipsometry. Unexpectedly, we showed that epitaxial Ti-rich BTO (with high Ti/Ba>10) can still be grown on GoS with perovskite structure but with altered crystal symmetry and quality against the perfect stoichiometric BTO layers. Finally, electrical properties of epitaxial BTO films on GoS are benchmarked in function of Ba/Ti ratio, highlighting the impact of the stoichiometry on the undesirable formation of oxygen vacancies in the layers.

Resume : Erbium (Er3+) is well-known for its transitions used in telecommunications systems due to the Er3+ intra-4f emission which corresponds to the 4I13/2 → 4I15/2 transition. Therefore Er3+ ions doped photonics materials are candidates for fabrication of optical amplifiers or lasers operating at 1530 nm. Lithium niobate is a very often used photonics material because of outstanding properties of its perovskite structure, which allows for modulation and amplification of the optical radiation at the same time. However, common single crystalline lithium niobate is in fact congruent crystal and real ratio of lithium to niobium is not 1:1 but about 6 % of lithium is missing. This fact enables easy doping with for instance magnesium but also, as in our case, with erbium. Doping of erbium into lithium niobate has been studied by our group for several years. In this contribution the summary of the erbium doping techniques into lithium niobate will be shown. It means that various techniques like erbium doping from metal or oxide layers, doping from melted erbium salt, erbium ion implantation, PLD and sol-gel techniques will be compared and also discussed. Possibility of erbium doping via ion implantation into other photonic crystals such as Al2O3, ZnO and diamond will be also mentioned. Properties of prepared thin films will be compared especially in terms of the erbium positions in crystal structure and luminescence properties. We acknowledge the Czech Science Foundation project GA 14-05053S. Some parts of this research were realised at the CANAM (Center of Accelerators and Nuclear Analytical Methods) infrastructure.

Resume : Olsen cycles are able to produce electricity thanks to phase transitions between states of different polarity in pyroelectric materials. Some perovskite compounds exhibit such phase transitions in the range 80-130?C, making them attractive for conversion of waste heat into electricity. Up to now, such systems work at lab scale with thin films of pyroelectric materials sandwiched between electrodes and subjected to varying electric field and bath temperature following Olsen cycles. Based on patent WO 2011/055065, we study here a new concept of pyroelectric conversion using concentrated suspensions of submicronic particles circulating through elements of given electric fields and temperatures reproducing the Olsen cycles. If this system may be expected as less efficient per cycle as compared to solid systems, it is also much more likely to be implemented in industries. Our choice was focused in the families KNbO3-PbTiO3 (KNPT) and KNbO3-BaTiO3 (KNBT). The challenges consisted in optimizing particle size and liquid phase to get the desired charge transfer effect and stable colloidal suspensions. KNPT particles in polyols were found promising and some electrical measurements will be shown.

Resume : Perovskite manganites are regarded one of the main building blocks of the oxides spintronics devices because of high spin polarization of the charge carriers and Curie point above room temperature. These properties are due to delocalization of the eg electrons in the network of 3d(Mn) and 2p(O) orbitals induced by the double exchange mechanism, leading to ferromagnetic order and metallic conductivity. However, this strongly correlated system displays a tendency to charge localization and phase separation (PS) into coexisting ferromagnetic/antiferromagnetic and metallic/insulating phases, having a detrimental impact on the magnetotrasport properties of these materials. Until now it has been accepted that in case of thin films of optimally doped La2/3Sr1/3MnO3 , phase separation takes place only within a narrow nanometrically thick dead layer close to an interface with a substrate. In this work we show that this picture is oversimplified. We have taken advantage of the extreme sensitivity of the 55Mn NMR technique to disentangle two different origins of phase separation in manganite (La2/3Sr1/3MnO3/SrTiO3) thin films. We observed that phase separation is not restricted to the dead layer at the interface, but propagates across the entire film thickness displaying a depth dependent density profile of localized holes (Mn4+). This is the evidence that the hole trapping centers located within the film volume play also an active role in PS. Moreover, from the analysis of the NMR restoring field sensed by the spins of 55Mn nuclei it has been found that these two distinct hole localization mechanisms act also as independent sources of magnetic anisotropy and their respective contribution to overall anisotropy varies across the film thickness.

Resume : Just as their standard counterpart, layered perovskite promise to be a interesting player in the field of multiferroicity. We will discuss, based on ab initio calculations, a few examples of exotic behavior of materials in the family AnBnO3n+2x for n=4 and 5. For the insulating n=4 case, we have two examples: LaMnO3.5 is a weak ferromagnet with an anomalously large linear magnetoelectric coupling, due mainly to the soft modes involved; LaTiO3.5 doped with V is a ferromagnet, and magnetization is inverted upon polarization inversion - the quintessential magnetoelectric effect. For the n=5 case, we find that BiTiO3.41 exhibits metallic character in one space direction coexisting with non-zero polarization in an orthogonal direction, a hybrid behavior opening the way to interesting potential applications.

Resume : A growing concern regarding the toxicity in lead-containing devices, has triggered a considerable interest in lead-free piezoelectrics. Because of its electronic similarity with Pb2+, the “lone-pair” cation Bi3+ appears as a natural replacement candidate and can produce a similarly large ferroelectric polarisation. The majority of studies have focussed on only two stable materials: the multiferroic bismuth ferrite BiFeO3 with a reported strain-driven MPB in thin films, and Na0.5Bi0.5TiO3 (NBT) which presents a high piezoelectric response when doped with Ba2+. Here, we will first discuss the structural richness of Bi-based materials [1,2], before presenting in more detail the structural features of Na0.5Bi0.5TiO3 (NBT) for which we report a temperature-dependent 3D atomic-level model, which is based on a neutron pair-distribution analysis [3]. Our statistical analysis of this model shows how local distortions compete, how this competition develops with temperature and, in particular, how different polar displacements/rotations of Bi-cations coexist, highlighting the interest of Bi-based materials in the search for new lead-free piezoelectrics. [1] M. Guennou et al. Phys. Rev. Lett. 112, 075501 (2014) [2] M. Guennou et al. Phys. Rev. B 84, 174107 (2011). [3] D. S. Keeble et al. Adv. Funct. Mat. 23, 184 (2013).

Resume : Influence of oxidation and reduction reaction in congruent LiNbO3 monocrystals have been analyzed in macro- and nano-scale with use of surface sensitive studies. All reduced samples of lithium niobate changed their color from transparent to black. Termogravimetry measurement has been conducted in low oxygen partial pressure 10-10 mbar in range from 500°C - 1000°C. Obtained results have shown mass loss in the crystal samples. Marked decrease in mass loss has occurred at 1000°C for redox reaction. Effusion experiments carried out at 800°C and 900°C under vacuum condition allowed us to determine type of released component, i.e. lithium oxide. The LiNbO3 monocrystals, have been reduced in ultra strong condition at 950°C, 850°C, 750°C and has been analyzed with use of the Secondary Ion Mass Spectroscopy technique. The 3D profiles have shown a non homogeneous distribution of lithium and niobium elements in the matrix of the sample. Electrical measurement has been conducted at 400°C in vacuum, in macro-scale (4 point technique, allows to define resistivity dependence from bulk, anode, and cathode area, simultaneously) and nano-scale (LC-AFM). Reduction process has induced heterogenic changes of resistivity in surface layer. Topology scans show conductive nanostructures like filaments and islands. XPS measurements have shown that marked changes have been observed at the surface layer and not in the interior part of the crystal.

Resume : Ferroelectricity in ABO3 perovskites and related compounds has been a topic of intensive research over the last 60 years. Recently, the coupling of the ferroelectric lattice mode with other non-polar modes has attracted an increasing interest since it offers promising and still widely unexplored possibilities to couple ferroelectricity with other functional properties and even to produce unusual phenomena. In this context, the trilinear coupling between ferroelectric and oxygen rotational modes in naturally-occuring and artificially layered perovskites emerged as a practical way to produce unusual dielectric properties or to achieve enhanced magneto-electric coupling. Focusing on vanadates and half-doped titanates, we will discuss here how even more interesting phenomena can appear when additional orbital and charge orders enter into play!

Resume : Transition metal perovskites have attracted huge interest over the last decades since the seminal discoveries of colossal magnetoresistance and high-temperature superconductivity. Vanadium based perovskites (A3+V3+O3, A=La to Y), although not widely studied, are a fascinating playground for modern physics. These systems are Mott insulators and crystalize in the well-known Pnma phase at room temperature. With decreasing temperature, they develop “Jahn-Teller” distortions modifying the degeneracy of the d orbitals and producing different orbital-ordered phases, which can be exclusive or coexisting. Additionnaly, those orbital ordered phases are intimately connected to particular antiferromagnetic orderings. Based on a symmetry mode analysis, we identify new relevant couplings between structural, electronic and magnetic degrees of freedom in vanadates and clarify the unusual coexistence of Jahn-Teller distortions, providing new insight on the latter and their role in perovskites in general. Using first-principles calculations, these new lattice mode couplings are proven to enable an unprecented Jahn-Teller and orbital ordering induced ferroelectricity in vanadate superlattices. Finally, due to the intimate connection between orbital ordering and magnetism, an electric field control of magnetism is predicted, opening novel possibilities for the design of magnetoelectric multiferroics.

Resume : Ferroelectric and ferroelastic domain walls (DWs) are becoming the focus of renewed excitement. Modern experimental techniques permit an unprecedented control on nano-domain structures, and it is now possible to produce materials with a large volume fraction occupied by the DWs themselves. Also, recent experiments show that DWs can display distinct properties not present in the domains. For example, measurements of increased conductivity and photovoltaic activity in materials like BiFeO3 suggest that the DWs could serve as active elements for applications in electronics. As a result, a lot of effort is being devoted to the engineering and optimization of the DWs. Understanding the novel DW behaviors remains a great challenge, and atomistic first-principles theory is called to play a key role in the field. We have employed a variety of tools, ranging from accurate quantum methods to approximate model potentials for large-scale simulations, to investigate DW-related phenomena. In this talk I will present some of our most striking results, including the discovery of novel structures and structural transitions at the DWs of well-known (multi)ferroic oxides, the tuning of DW properties with epitaxial strain, and ideas for inducing novel orders confined at the walls.

Resume : PbZr1-xTixO3 (PZT) system has been extensively studied over the past decades for both industrial applications and fundamental research. Recently, PbZr1-xTixO3 single crystals with compositions (0.2 ≤ x ≤ 0.65) across the mophotropic phase boundary (MPB) region have been grown in our laboratory by a top-seeded solution growth (TSSG) technique. The availability of PZT single crystals makes it possible to systematically investigate the correlation between structural evolu-tion and electrical properties in PZT system. In this work, the domain structure and phase transitions of PZT single crystals with compositions x=0.32, x=0.42, x=0.46 and x=0.62 are studied by polarized light microscopy (PLM), piezoresponse force microscopy and dielectric measurements. PLM observation shows an in-crease of extinction angle in the two rhombohedral crystals: x=0.32 at 340 °C, and x=0.42 at 250°C, indicating a thermally induced polarization rotation accompanied with the rhombohedral (R) to monoclinic (M) phase transition. The ferroelectric to paraelectric phase transition in x=0.32 is of first order with a discontinuous change of birefringence around TC=351°C. The dielectric measurements show anomalies below TC, confirming the R-M phase transitions in both crystals. For the MPB composition x=0.46, PLM shows a monoclinic symmetry with complicated domain at room temperature, which upon heating changes to a tetragonal phase at about 286°C and then to a cubic phase at TC=394 °C. Crystals of compositioPbZr1-xTixO3 (PZT) system has been extensively studied over the past decades for both industrial applications and fundamental research. Recently, PbZr1-xTixO3 single crystals with compositions (0.2 ≤ x ≤ 0.65) across the mophotropic phase boundary (MPB) region have been grown in our laboratory by a top-seeded solution growth (TSSG) technique. The availability of PZT single crystals makes it possible to systematically investigate the correlation between structural evolu-tion and electrical properties in PZT system. In this work, the domain structure and phase transitions of PZT single crystals with compositions x=0.32, x=0.42, x=0.46 and x=0.62 are studied by polarized light microscopy (PLM), piezoresponse force microscopy and dielectric measurements. PLM observation shows an in-crease of extinction angle in the two rhombohedral crystals: x=0.32 at 340 °C, and x=0.42 at 250°C, indicating a thermally induced polarization rotation accompanied with the rhombohedral (R) to monoclinic (M) phase transition. The ferroelectric to paraelectric phase transition in x=0.32 is of first order with a discontinuous change of birefringence around TC=351°C. The dielectric measurements show anomalies below TC, confirming the R-M phase transitions in both crystals. For the MPB composition x=0.46, PLM shows a monoclinic symmetry with complicated domain at room temperature, which upon heating changes to a tetragonal phase at about 286°C and then to a cubic phase at TC=394 °C. Crystals of composition x=0.46 exhibit the best piezoelectric properties, with a piezoelectric constant d33=1223 pC/N and an electromechanical coupling factor k33=80%. The (100) plate of the tetragonal crystal with composition x=0.62 is optically isotropic, but the (011) plate shows a small birefringence at 45° (angle between polarizer and <100> direction). This is because of the formation of nano-sized tetragonal domains due to the instability of the ferroelectric phase. A total compensation is achieved in the (100) plate, but in the (011) plate the compensation is only partial. By applying an electric field, macroscopic domains can be induced in a (100) plate at a field of 25kV/cm and the crystal becomes aniso-tropic. Birefringence becomes larger with increasing field and shows a maximum value Δn=0.012 at the field of 30kV/cm. After the electric field is removed, the crystal changes back to isotropic, indicating the field induced macroscopic domain is metastable. We discuss the mechanisms for the formation of domain structures and phase transitions in PZT single crystals and compared them with the behaviour and mechanisms of relaxor ferroelectrics.

Resume : PbZr1-xTixO3 (PZT) is one of the most important and widely used piezoelectric materials. The study of its local and average structures is of fundamental importance in understanding the origin of its high-performance piezoelectricity. Pair Distribution Function analysis and Rietveld refinement have been carried out to study both the short- and long-range order in the Zr-rich rhombohedral region of the PZT phase diagram. The nature of the monoclinic phase across the Zr-rich and morphotropic phase boundary area of PZT is clarified. Evidence is found that long-range rhombohedral and both long- and short-range monoclinic regions all coexist at all compositions. In addition, evidence for a boundary between the R and M regions has finally been found and is shown to be characterized by a change from one type of monoclinic (MA) to another monoclinic (MB) structure.

Resume : The main lattice instabilities of PbZrO3 arise from Pb and O atoms shifts, which induce, in the antiferroelectric phase, a modulation in the ab plane and oxygen tilts. We studied the polarized IR and Raman spectra of oriented single domain plates and the temperature dependence of phonons. We identified separately all symmetry modes and found soft modes of seven different symmetries in the antiferroelectric phase. This can be understood in the frame of a three dimensional lattice model with trilinear coupling terms allowed by symmetry, which can stabilize the antiferroelectric displacements of Pb, within a matrix with tendency to the long range ferroelectric order. [1] The substitution of a small percent of Zr by Ti in this system (1% of Ti) induces very interesting phenomena. We also studied PbZr0.99Ti0.01O3 single crystals by means of micro-Raman scattering and Second harmonic generation. The crystals undergo two first order phase transitions within the approximate temperature range 488?503 K. The presence of the second harmonic signal within the phase transitions, together with the analysis of the Raman spectra, leaded us to the conclusion that the intermediate phase is ferroelectric and, probably rhombohedral with the space group R3m. This result support the model sketched in [1] for the development of the antiferroelectricity in PbZrO3. Support by the Czech Science Foundation is acknowledged (Project 13-15110S). [1] J. Hlinka et al., Phys. Rev. Lett 112, 197601 (2014)

Resume : The development of piezoelectric materials with working temperatures > 700 K comprises actual task due to demands of industries and ecological problems. Ceramic solid solutions with based on oxides with perovskite structure (Bi,Pb)(Sc,Ti)O3 and (K0.5,Na0.5)O3 attract much attention due to prospects of their applications at high temperatures. In this work, influence of preparation conditions and various overstoichiometric additives (oxides, chlorides and fluorides in amounts up to 15 w. %) on structure, microstructure, ferroelectric and piezoelectric properties of ceramics (1-x)BiScO3 – xPbTiO3 with x=0.635-0.65 (BSPT) and (K0.5,Na0.5)O3 with compositions close to the Morphotropic Phase Boundaries were studied using X-ray Diffraction, Scanning Electron Microscopy, Second Harmonic Generation, and Dielectric Spectroscopy methods. Piezoelectric parameters d33 and kt of the preliminary poled samples were measured. Optimal conditions for dense and textured ceramics preparation were determined, changes in the phase content and unit cell parameters stimulated by additives observed. The 1st order ferroelectric phase transitions were revealed at temperatures near 700 K. High piezoelectric coefficients d33 ~ 500 pC/N and kt ~ 0.65 were reached in modified BSPT ceramics. Enhancement of piezoelectric properties is discussed in relation to the preparation conditions, type and content of dopants. The work was supported by the RFBR grant 12-03-00388.

Resume : Many functional materials are traditionally synthesized by slow reaction processes that are energy and time consuming. In the present world there is strong demand on development of modern materials and materials processing methods that could offer rapid reaction rates, energy efficiency and be environmentally safe. Perovskite oxide ceramics have found wide applications in energy storage capacitors, electromechanical transducers and piezoelectric, and ferroelectric devices. The conventional method to prepare Perovskite oxide ceramics is ceramic-powder-based processing, i.e., through solid-state reaction at high temperatures. This process has several disadvantages, such as high-temperature reaction, limited degree of chemical homogeneity, and low sintering ability. Therefore, during past years, several chemical-based processing routes, including freeze-drying, spray-pyrolysis, sol– gel, spray-drying, and pyrolysis of complex compounds, have been developed to prepare powders with more homogeneous composition, improved reactivity, and sintering ability at low temperatures. Recently, non-conventional processing methods such as mechanical alloying and mechano-chemical approaches have been used to create reactions between species. However in this method the reaction kinetics is very slow and processing time long. Here we report application of an Electric Discharge Assisted Mechanical Milling (EDAMM)[1] technique to synthesize various Perovskite oxide ceramics. By using EDAMM, high purity single phase multi-element oxides can be formed in as little as 0.1% of the processing time required in conventional solid-state techniques. An even more important feature of EDAMM is that the crystallite size of the synthesized compound is able to be reduced to nanometer size , by careful selection of electrical (voltage, current, total power) and mechanical (vibration frequency and amplitude) experimental parameters. We use EDAMM for (i) synthesis of oxides from elemental powders by oxidation in oxygen plasma and for (ii) synthesis of single phase multi element oxides from pre-mixed oxides as starting materials. This presentation provides an overview of recent development of EDAMM method and it’s application in rapid synthesis of Perovskite ABO3 type oxide ceramics. The EDAMM technique offers an exciting opportunity to rapidly synthesize a range of new and existing materials to be used in a variety of energy storage applications that include rechargeable lithium batteries, hydrogen fuel cells, and super-capacitors. [1] Nature, 419,(2002)147-151