Ferroic perovskites for advanced materials

Resume : Barium titanate (BT) is a classic ferroelectric with average properties. However, they can be improved in two ways – by either introduction of other ions into the lattice, or by making a composite material. Morphotropic phase boundary is the goal in the first case, but it is not always present in different combinations of materials. The second case is less demanding, but requires epitaxial interfaces between the different regions of the composite. If Potassium niobate (KN) is inserted into the BT structure and forms a composite with epitaxial interfaces, then stresses are created, that increase domain wall count and as a result piezoelectric coefficient. Although ferroelectric Pb(Zr,Ti)O3 has taken a substantial share of piezoelectric market due to its high piezoelectric characteristics, BT-KN composite ceramics have a potential to become a substitute. The fact that BT-KN is lead free ceramic makes it of high interest for both researchers and engineers due to environmental concerns. In this presentation dielectric properties of BT-KN with KN molar ratios of 0.22, 0.25 and 0.5 will be presented. It was experimentally found that there is a strong contribution of piezoelectric effect to the effective permittivity of the composites. The characteristic frequency of the process depends on mean cluster size of the barium titanate particles. Furthermore, adjacent grains have correlated direction of net polarisation of the grains.

Resume : In addition to resistor, capacitor and inductor, Chua proposed in 1971 the fourth fundamental circuit element to directly relate magnetic flux and charge. Such a circuit element, dubbed memristor, was later attributed to the non-linear current-voltage characteristic and has been realized in various material structures. Here we clarify that a two-terminal device employing magnetoelectric effect possesses the function of relating directly magnetic flux and charge, and fulfill the definition of the fourth circuit element. Both the linear and the nonlinear memory devices having this function, dubbed transtor and memtranstor, respectively, are realized by sandwiching a magnetoelectric hexaferrite crystal in electrodes. Moreover, the devices can operate either in the charge-driving mode or in the flux-driving mode, and the transdance arising from the magnetoelectric coefficient can be both positive and negative, thus displaying a pinched hysteresis loop with special shape. With the introduction of transtor and memtranstor, a complete and harmonized relational graph can be constructed for the fundamental circuit variables, which will be helpful for broadening circuit functionality for next-generation intelligent devices in future.

Resume : Doping ferroelectric compounds to produce novel multiferroic materials is a seductive approach. We report on the Fe doping and on the comparative NiFe2 co-doping of fully oxidized BaTiO3 layers (~ 20 nm) elaborated by atomic oxygen plasma assisted molecular beam epitaxy. The films were thoroughly characterized by synchrotron radiation X-Ray diffraction and X-Ray absorption spectroscopy. For purely Fe doped layers, the native tetragonal perovskite structure evolves rapidly towards cubic-like up to 5% doping level above which the crystalline order disappears. On the contrary, low co-doping levels (~5%) fairly improve the thin film crystalline structure and surface smoothness; high levels (~27%) lead to more crystallographically disordered films, although the tetragonal structure is preserved. Magnetic dichroic measurements reveal that metal clustering does not occur and allowed determining the ion valences. Ferromagnetic long range ordering can be excluded with great sensitivity in all samples as deduced from X-Ray magnetic absorption circular dichroic measurements. On the contrary, our linear dichroic X-Ray absorption results support antiferromagnetic long range ordering while piezo-force microscopy give evidence of a robust ferroelectric long range ordering showing that the films are excellent candidates for magnetic exchange coupled multiferroic applications.

Resume : It has been recently proposed that it should be possible to introduce ferromagnetism and ferroelectricity in SrMnO3-related perovskites when large strains are achieved. We have predicted earlier that similar to the d0 titanates such strain should be observed in the non-d0 perovskites resulting in a strong ferroelectric-magnetic coupling of the same magnetic ion. By advancing elaborate synthesis processes, which are necessary to avoid the more stable hexagonal polymorphs, we were recently able to extended the substitution limit of the large size Ba ion to prepare highly strained multiferroic Sr2+1-xBa2+xMn4+O3 perovskite ceramics (x=0.4-0.45) for which ferroelectricity (TF~300-350 K) and antiferromagnetism (TN~200 K) originate exclusively from the Mn (d3) cations. Similar to Ba2+Ti4+O3, the classical displacive-type ferroelectric phase transition occurs when the Mn ions move out of the center of the MnO6 octahedral units. The largest known magneto-electric coupling was observed near TN when ferroelectricity disappears. Substitution of Ti for Mn enhances ferroelectricity and increases TF to above 400 K while suppresses antiferromagnetism and lowers TN to below 200 K. As a result it is possible to tune the magnetoelectric coupling and preserve the ferroelectric phase below TF; i.e., to achieve unique displacive-type multiferroic with large spontaneous polarization.

Resume : Photoluminescent layered oxides doped with lanthanides are always attracted to scientific community. Layered oxide materials are important because of their physical and chemical stabilities and also with the photoluminescence of lanthanides with well-defined 4f transitions in the host layer. There are many examples of photoluminescent lanthanide doped oxide materials. These materials are generally 3D bulk crystals or amorphous materials. Layered oxides are unique with their layered structure and have distinctive properties. Layered oxides can be exfoliated into their 2D nanosheets. These nanosheets can absorb light efficiently because of their planar surface and high aspect ratio. Ultra thin films can be deposited in a controlled manner by using these nanosheets. Different classes of nanosheets doped with different lanthanide cations can be utilized in order to fabricate thin composite films emitting light at different wavelengths. Layered perovskite oxides are intergrowths of perovskite and other structures. They consists of two-dimensional perovskite sheets separated with cations or cationic units. They are classified as Dion-Jacobson, Ruddlesden-Popper and Aurivillius phases. The A site in the perovskite layer can be exchanged with another cation which could be s-, p- or f- block cation. In general, f-group cations will be emission centers and perovskite will emit at characteristic wavelengths of the f-block element as a result of well-defined f-f transition. Nanosheets doped with f-block elements can easily be prepared from bulk layered oxides by using wet-chemistry. I will discuss the optical properties of lanthanide doped Aurivillius and Dion-Jacobson phase layered perovskite materials. Harmonic oscillation and upconversion examples of these type of layered oxides doped with lanthanides will be presented. I will also show some examples of photocatalytic properties of perovskite type layered oxides. We would like to acknowledge TUBITAK for funding through project 114Z452 (Synthesis of two dimensional luminescent perovskite nanosheets).

Resume : Oxygen vacancies in ABO3 type perovskites play an important role for many applications. Using hybrid DFT calculations combined with LCAO basis set and periodic supercell approach, the atomic, electronic structure, and phonon properties of both neutral and fully charged oxygen vacancies in several perovskite oxides (SrTiO3, LaFeO3, LaCoO3 and BaZrO3) were calculated and compared. An emphasis is given to the analysis on the role of 3d-electrons in determining the system electronic structure and the presence of so-called “ghost” basis functions centered at the vacant site. It is shown that the Gibbs free formation energy (Gf) of neutral oxygen vacancy is smaller when a basis set is present at the vacant site than without the basis functions. The electronic charge concentrated within a neutral vacancy varies from 0.5 e (SrTiO3) to almost 1.0 e (BaZrO3), indicating a considerable effect of the electron localization and depending on the position of defect level in the band gap. Also, the contribution of phonons to Gf in BaZrO3 is shown to be much larger for the charged vacancy in a comparison with the neutral vacancy whereas it is still large for the neutral vacancy in SrTiO3 due to the temperature dependence of soft modes. We present a careful comparison of the obtained results with the experimental data for the optical properties, electrical conductivity and thermogravimetry measurements.

Resume : There is an increasing interest in the investigation of defects formation and properties in multiferroics, as they are believed to play a key role in enhancing transport properties [1,2] (to the point of superconductivity in WO3 [3]), or creating new complex distribution of the magnetic/electric degrees of freedom (e.g. vortices [4]). The impact of defects on pinning domain walls [5,6], fatigue and imprint, remains to be exhaustively studied in order to understand how defects can deteriorate/enhance material functionalities. We studied the occurrence of single and bi-defects in Bismuth Ferrite using ab-initio calculation with ABINIT, both in GGA and LDA+U, and show that the necessary corrections to achieve the study of defects in the dilute limit yield similar results. In addition, we show that single point defects will tend to aggregate, with a binding energy for the Bi-O pair of approximately 0.15 eV. Finally, we study the possibility to engineer defects by using different atmospheres in oxygen. Acknowledgements References [1] S. Farokhipoor, B. Noheda, Phys. Rev. Lett. 107, 127601, (2011). [2] J. Seidel, L. Martin, Q. He et al., Nat. Mat. 8, 229, (2009). [3] E. Salje, Chem Phys Chem 11, 940, (2010). [4] D. Viehland and E. Salje, Adv. Phys. 63, 267, (2014). [5] L. He and D. Vanderbilt, Phys. Rev. B 68, 134103 (2003) [6] A. Chandrasekaran et al., Phys. Rev. B 88, 214116 (2013)

Resume : Rare-earth bismuth-containing manganites with a perovskite structure relate to the perspective multifunctional materials of spintronics. In these materials the polarization effects of magnetical and ferroelectric states manifested in the coexistence of magnetoresistance (MR) and ferroelectric (FE) properties are observed. The possibility of focused changes and management of these properties due to changes in the composition, structure and its defects determines the relevance of the research. Single-phase ceramics La0.6Sr0.3-xBixMn1.1O3-δ (x=0;0.1;0.15;0.2) obtained by the standard solid-state reaction was investigated by X-ray, thermogravimetric, resistive ρ, magnetic, 55Mn NMR, SEM and MR methods. The substitution of Sr2+ ions by bismuth leads to an increase in the parameter of the R3 ̅c perovskite structure, low temperatures of “metal-semiconductor” and “ferro-paramagnetic” phase transitions, as well as reducing peak temperature of MR-effect. The part of bismuth ions are in the B-position as Bi5+. The increase of the activation energy and a sharp increase in ρ indicate the approach of a FE-state. Defective molecular formulas of the real perovskite structure containing anion and cation vacancies were determined. The analysis of 55Mn NMR spectra confirmed the conclusion about the manganese ions environment heterogeneity by vacancy-type defects. The constructed phase diagram shows a strong correlation between the defect structure and the properties of ceramics investigated.

Resume : The first-principles total energy calculations of BiAlO3 and BiGaO3 in different crystal phases under pressure up to 15 GPa have been performed within generalized gradient approximation (GGA). The calculations were done within projector-augmented wave method as implemented in VASP. At 10 GPa BiAlO3 undergoes a structural phase transition from rhombohedral R3c phase to orthorhombic Pnma phase. This phase transition is from ferroelectric to paraelectric phase. The calculated spontaneous polarization of rhombohedral BiAlO3 is 75 μC/cm2. In case of BiGaO3 the structural phase transition appears at 3.5 GPa from pyroxene-like structure (Pcca space group) to perovskite-like structure (Cm space group). The latter is paraelectric-ferroelectric phase transition. The calculated spontaneous polarization of BiGaO3 Cm phase is 130 μC/cm2. This work was supported by the National Science Centre (Poland) through the Grant no DEC-2011/01/B/ST3/02212.

Resume : Barium titanate and bismuth ferrite are the most extensively studied multiferroic materials. They are of high interest mainly due to simple perovskite crystallographic structure and possibilities of applications in non-volatile magnetoelectric devices. Nevertheless, ferroelastic properties of these crystals cannot be easily exploitable, because the reversal of electric polarization by electric field is not accompanied by a change of spontaneous strain. The strain-mediated magnetoelectric coupling is reached in this structures usually via piezoelectricity. On the other hand, ferroelasticity in Gd2(MoO4)3 (gadolinium molybdate) could be exploited more easily, since it is a full ferroelectric- full ferroelastic crystal. Hence, the reversal of polarization by electric field induces change in spontaneous strain there. The permanent spontaneous strain will stabilize magnetic information in multiferroic heterostructures more effectively. Here we report on the first study devoted to the thin film stabilization of GMO and the study of its epitaxial relationship on strontium titanate (SrTiO3) and silicon (001) substrates. We studied the influence of substrate temperature on the crystalline properties of the films. Morphology and epitaxial relationship were characterized with XRD, XRR, HR-TEM and AFM techniques. Results show that epitaxial strain provided by the STO(001) substrate induces the stabilization of a highly oriented Scheelite-type structure GdMoO4(001), while Si(001) allows the growth of Gd2(MoO4)3 in polycrystalline manner.

Resume : A comparative study of ferromagnetic double perovskites of R(Ni0.5Mn0.5)O3 (R=Sm, Nd, Pr, La, Bi & Y) is presented. Films were carefully deposited by PLD on STO(001) substrates, aiming to stabilize a fully textured and high quality films. The epitaxial, cube on cube, single phase stabilization of the Gd & Nd perovskites was achieved, however, in the case of Sm and Y, the single phase stabilization of the films showed to be challenging. We compared both the ionic radius of the Re atoms and the lattice constants of all the RNMO perovskites, both in bulk and thin films. As a result it was shown that RNMO structure presents a limit in which the tetragonality can be induced in the unit cell, thus allowing a cube on cube epitaxial growth with different levels of epitaxial strain. The comparison of the ratio between lattice constants in the perovskites shows a critical value of b/a= 1.025 above which, the studied materials start showing multiple in-plane and out-of-plane orientations, for Ytrium and Samarium respectively. Finally, magnetic measurements and atomic force micrographs of the deposited films are shown.

Resume : The last decade saw little progress on the research on the CaO-Li2O-Sm2O3-TiO2 (CLST) microwave dielectric ceramics due to the complexity in experimental based mechanistic research on dielectric properties. It is mainly influenced by both intrinsicand extrinsic factors. The present study uses the electronic structure calculation with crystal models to evaluate the intrinsic influences of A-site substitutions (Mg2 , Ba2 and Sr2 ) on the dielectric properties of CLST ceramics. In addition, CLST ceramics are prepared using a solid-state reaction and characterized. The electronic structure calculation shows that Mg2 or Ba2 enhances the covalent bond of the crystal, reducing the dielectric loss and permittivity, while Sr2 does the opposite. The experimental results show that Sr2 enters the lattices within the range of substitution amounts, while Ba2 does so only with low substitution content. In these situations, the variation tendencies of the properties with the substitutions agree to the prediction of the electronic structure calculation well, suggesting that intrinsic factors play the main role on the dielectric properties. On the other hand, Mg2 substitution and high content of Ba2 substitution are unable to form homogenous solid solution, since the ion sizes are too small (for Mg2 ) or too big (for Ba2 ), but second phases are formed instead. In these situations, the variation tendencies of the properties with the substitution agree to the two-phase composition rule.

Resume : The solid solution (1-x)Pb(Ni1/3 Nb2/3)O3-xPbTiO3 (PNN-PT) exhibits relaxor to purely ferroelectric behaviour with large piezoelectric coefficients near the so-called Morphotropic Phase Boundary (MPB) by increasing the PT amount. This is for example the case for many other relaxor-PT systems like PMN-PT, PZN-PT, ... Here, the presence of Ni cations enriches this solid solution by providing potential magnetic and optical features making PNN-PT a multifunctional perovskite oxide. Therefore the structural, electronic, dielectric, piezoelectric, optical and magnetic properties were systematically investigated on a broad range of compositions, ranging from x=0 to x=0.6. We show indeed strong relaxor-like properties (dielectric relaxation, electrostriction) and large piezoelectric coefficient reaching 1000 pC/N at room temperature near the MPB at x=0.35. The optical properties (reflectance, band gap, ...) reveal the absorbance potentialities of PNN-PT in the visible and UV range. In addition, the study of the lattice constant with temperature, coupled to calorimetric experiments, point to a possible Jahn-Teller distortion related to low to high spin transition of the Nickel ion near room temperature for solid solution with low PT content.

Resume : In contrast to ferroelectrics, antiferroelectrics have received less attention despite rich and still yet understood phenomena making them of interest for academic research as well as for potential applications including high-energy storage capacitors, electrocaloric devices, or sensors and transducers. Several phonon mode instabilities are known to exist in antiferroelectrics like in the prototypal PbZrO3 (PZ) system. Such instabilities imply antiparallel and/or ferroelectric displacements as well as in-phase and/or antiphase oxygen octahedra tiltings. It is believed, but still debated, that these modes compete and/or may couple leading to antiferroelectric behavior. By destabilizing these modes, it is then possible to get various polar states. As examples, PZ goes through a peculiar ferroelectric state with temperature increasing prior to the paraelectric state or Pb(Yb1/2Nb1/2)O3-PbTiO3 (PYN-PT) solid solution which can exhibit all polar orders at room temperature, thus leading to antiferroelectric, classical ferroelectric, or relaxor properties increasing PT amount. While the behavior of antiferroelectrics was recently reconsidered, only few works using local probes to reveal the polar features have been published. In this work, we use piezoresponse force microscopy (PFM) to get insight into the local features of induced polar arrangements as a function of electric field and temperature in these systems. After recalling structural and electric features on the above mentioned compounds deduced from larger scale techniques including X-ray diffraction and neutron scattering, Raman and dielectric spectroscopies, as well as macroscopic piezoelectric and ferroelectric measurements as a function of temperature we will reveal the local phenomena underneath these properties seen by PFM. As a result, we evidence on PZ single crystals the arising of a piezoelectric response through the antiferroelectric to ferroelectric phase transformation and its time-dependent evolution by performing PFM mappings as a function of temperature. On PYN-PT ceramics of various compositions, we show the room temperature studies, including local piezoelectric loops to explore compositions with low PT content near the antiferroelectric end-member (0≤x≤0.16) and direct imaging of virgin and induced domain patterns characteristic of relaxor, ferroelectric-relaxor, and ferroelectric materials (0.20≤x≤0.60). In the latter case, we will confirm and complete with these PFM studies the PYN-PT phase diagram, showing the antiferroelectric order from local piezoelectric loops, and studying for virgin and artificial polar domains the coherence lengths and the corresponding temperature transition, respectively. These results will be presented within the global picture of the piezoresponse microscopy studies comparing antiferroelectrics with relaxors and ferroelectrics.

Resume : Early measurements by Courtens [1] suggested that the birefringence in the ferroelastic phase varied with temperature with a critical exponent for the order parameter of 0.36(2). More recent measurements [2], in which birefringence imaging was used to make sure that a domain-free region of the crystal was measured, revealed a mean-field exponent close to 0.5. It was suggested that Courten’s measurement, which was carried out by averaging over a field of view of the crystal, might have been affected by the presence of domains appearing within the low-temperature phase. More importantly for the purposes of this report, Courtens observed a curious cusp appearing just above the phase transition that he explained in terms of critical fluctuations. Such behavior was not seen in the work [2]. In this report we will show that this cusp can be avoided if only phase transition is observed at no change in the indicatrix orientation during structural transformation (Fig 2). At the same time the temperature behavior of birefringence below TS is not pointing to purely displacive type transition. Above Ts the precursor dynamics is present [3] independently on crystal technology and its temperature range is in good agreement with temperature range reported also for micro- and nanoceramics of SrTiO3 by Hehlen et al [4]. 1. E. Courtens, Phys. Rev. Letters, 29, No. 20, 1972 2. M. A. Geday and A. M. Glazer, J. Phys.: Condens. Matter 16, 3303–3310, 2004 3. K. Roleder, A. Bussmann-Holder, M. Górny K. Szot and A. M. Glazer, Phase Transitions, 85, No. 11, 939–948, 2012 4. B. Hehlen, A. Al-Zein, C. Bogicevic, P. Gemeiner, and J-M. Kiat, Phys. Rev. B87, 014303 (2013)

Resume : Ferroelectric Ba1−xSrxTiO3 (BST) ceramics are of great technological interest. Due to its large pyroelectric coefficient, high dielectric permittivity, low dielectric losses and high dielectric breakdown strength, the ferroelectric BST has recently been identified as promising candidate for an inexpensive infrared thermal imaging device called uncooled infrared focal plane arrays [1]. The BST, especially due to its high dielectric permittivity, may replace the current silicon oxide and nitride dielectrics to build up next generation of dynamic random access memories, which require higher integration densities [2]. In addition, the strong dependence of dielectric permittivity on electric field [3] makes them attractive for application in voltage-tunable microwave devices such as filters, tunable oscillators, delay lines, and phase shifters [4], which are essential elements in communication and radar systems [5]. The dielectric properties of BST can be modified not only by controlling the Ba/Sr ratio but also through partial substitution of iso- or heterovalent cations in A and/or B-sites in ABO3 perovskite structure [6, 7, and 8]. The influence of rare earth oxides such as Dy2O3, Er2O3 and Sm2O3 on the structural and dielectric properties of barium strontium titanate ceramics has recently been extensively investigated [9, 10, and 11]. However, information about La2O3-doped BST ceramics, especially of the temperature dependence of dielectric parameters, is scarce [12-17]. In this report the results of experimental studies of the influence of La2O3 on the grain structure, phase transitions and dielectric properties of (Ba0.8Sr0.2)TiO3 (BST 80/20) ceramics will be presented. The La concentrations in ceramics, which were prepared by conventional mixed oxide method, was changing from 0.5 to 2 mol. %. The unit cell parameters were determined from the x-ray diffraction measurements, and scanning electron microscope JSM 5410 with an energy dispersive X-ray spectrometer (EDS) was used to reveal the grain structure and chemical composition of ceramics obtained. Detailed studies of dielectric permittivity as function of temperature and frequency showed that these compounds undergo a diffuse phase transition. Measurements of the pyroelectric and thermally stimulated depolarization currents have confirmed character of this transition. References: [1] S. Liu, M. Liu, Y. Zeng, and Y. Huang, Mat. Sci. Eng. B90, 149, (2002). [2] A. I. Kongon, S. K. Streiffer, C. Basceri, and S. R. Summerfelt, Mater. Res. Bull. 46, (1996). [3] S. Lahiry and A. Mansingh, Ferroelectrics 306, 37, (2004). [4] F. Zimmermann, M. Voights, C. Weil, R. Jakoby, P. Wang, W. Menesklou, and E. Ivers-Tiffe, J. Eur. Ceram. Soc. 21, 2019, (2001). [5] S.-G. Lee, C.-I. Kim, J.-P. Kim, and S.-H. Lee, Mater. Lett. 58, 110, (2004). [6] L.Szymczak. L.Kozielski, M.Adamczyk, A.Lisińska-Czekaj, D.Czekaj, Z.Ujma, Ferroelectrics, 349, 179-189, (2007). [7] L.Szymczak, Z.Ujma, M.Adamczyk, M.Pawełczyk, Ceramics International, 34, 1993-2000, (2008). [8] L.Szymczak, M.Adamczyk, M.Pawełczyk, Archives of Metallurgy and Materials, 54(4), 943-949, (2009). [9] Huang X Y, Gao C H, Chen X C, Zheng X L, Huang G J, Liu H P., J Rare Earths, 22(z3): 226−228, (2004). [10] Jingji Zhang, Jiwei Zhai, Xiujian Chou, Xi Yao, Materials Chemistry and Physics, 111,409, (2008). [11] Li Y, Qu Y. J., Mater Res Bull, 44(1): 82−85, (2009). [12] Ling Dong, Xu Guo-yue, Cai Shaoa, J. of Optelectronics and Advanced Materials, 7(5), 2737 -2742, (2005). [13] RH Liang, XL Dong, Y Chen, et al., Materials Chemistry and Physics, 95(2-3), 222-228, (2005). [14] XH Wang, WZ Lu, J Liu, et al., J. Eur. Ceram. Soc., 26(10-11), 1981-1985, (2006). [15] Chunlai Xu, Heping Zhou, Key Engineering Materials, 336-338(1-3), 249-251, (2007). [16] Yanhua Fan, Shuhui Yu, Rong Sun, et al., Applied Surface Science, 255(20) 8319-8323, (2009). [17] C. R.Gautam, A. K.Yadav, P.Singh, Materials Research Innovations, 17(3), 148-153, (2013).

Resume : Ferroelectric oxides have conventionally developed with oxides mainly composed of oxygen-octahedra, which are represented by perovskite-type compounds such as BaTiO3, Pb(Zr,Ti)O3, BiFeO3, and so on. Though their various ferroic properties are fascinating topics in the field of physics and materials science, the discovery of new ferroelectrics in the octahedra-based oxides seems to be gradually saturating in these days. There is thus increasing demand for mining new ferroelectric systems by expanding the area of exploration. Our group recently focused on the development of new ferroelectric oxides, which are mainly composed of oxygen tetrahedra, in particular SiO4 and AlO4. An advantage of the oxygen-tetrahedra-based oxides is their environmental affinity and natural abundance, since most of rock-forming oxides in the earth?s crust are known to be formed by the oxygen-tetrahedra. There are however very few tetrahedra-based-oxides reported to show ferroelectricity, except for water-soluble phosphates and phosphites that are generally difficult to apply for modern electronics. Here I show the bismuth silicates, Bi2SiO5, whose ferroelectricity has recently been demonstrated by our group. Based on the mechanism of ferroelectricity clarified in Bi2SiO5, I will discuss the designing principle of new tetrahedra-based ferroelectric oxides by introducing some examples in ferroelectric stuffed-zeolites.

Resume : Over the last decade there has been an explosion of interest in sheet conductors, such as surface states in topological insulators [1], LaAlO3 -SrTiO3 interfaces [2] and graphene [3]. Recent research has shown that ferroic domain walls constitute another exciting group of 2D conductors, with probably even greater potential than those already known: after all, domain walls have special properties that other systems do not have in that they are mobile, can be controllably shunted from point to point, and can be spontaneously created, or made to disappear. Luckily for the research community, the field of ?domain wall nanoelectronics? [4] is still young and there is a great deal, in terms of fundamental science, still to be uncovered. This talk will concentrate on two areas of domain wall research recently performed at Queen?s University Belfast: firstly, experiments revealing fundamental insight into domain wall conduction mechanisms will be described. Specifically, the use of scanning probe microscopy to determine Hall voltages at domain walls will be discussed, as will the active carrier types, mobilities and densities implied in both rare earth manganites and metal-halide boracites (both improper ferroelectrics with transport-active ?head-to-head? and ?tail-to-tail? walls). Secondly, various ways in which domain wall injection and motion [5] have been controlled, using sample morphology, electric and stress fields, will be described which have culminated in the successful creation of a ferroelectric domain wall diode [6]. [1] H. Zhang et al. Nature Physics 5, 438 (2009) [2] A. Ohtomo and H. Y. Hwang, Nature 427, 423 (2004) [3] K. S. Novoselov et al. Science 306, 666 (2004) [4] G. Catalan et al. Rev. Mod. Phys. 84 119 (2012) [5] J. R. Whyte et al. Adv Mat. 26, 293 (2014); J. R. Whyte et al. J. Appl. Phys. 116, 066813 (2014) [6] J. R. Whyte et al. Nat. Comms (in press 2015)

Resume : Phase-field simulations demonstrate that the polarization order-parameter field in the Ginzburg-Landau-Devonshire model of rhombohedral ferroelectric BaTiO3 allows for an interesting linear defect, stable under simple periodic boundary conditions. This linear defect, termed here as Ising line, can be described as about 2 nm thick intrinsic paraelectric nanorod acting as a highly mobile borderline between finite portions of Bloch-like domain walls of the opposite helicity. These Ising lines play the role of domain boundaries associated with the Ising-to-Bloch domain wall phase transition.

Resume : Solid solutions of lead-based perovskites are often showing either a ferroelectric transition with a glassy dynamics or enhanced piezoelectric properties. The IR spectroscopy of lead-based relaxors is well understood[1], but the assignment of Raman spectra remains problematic. The Raman activity originates from both the occupational ordering [2] and the ionic off-centering [3]. The weight of these effects changes from one material to another. Morever, the inherent disorder lifts the strict Raman selection rules, while the Hyper-Raman scattering spectra obey the polarization selection rules rather well [4]. Still, Raman scattering in relaxors shows a measurable polarization dependence and specific features of Raman scattering can be even employed to probe relaxor to ferroelectric crossover [5] or to distinguish between distinct ferroelectric phases coexisting in materials with compostion close to the so-called MPB boundaries[6]. Here, our recent polarized Raman scattering studies of relaxors will be discussed. 1. JH et al, Phase Transitions, 79, 41 (2006). 2. Setter N and Laulicht I, Appl. Spectrosc., 41, 526 (1987). 3. Iwata M, et al, Jpn. J. Appl. Phys., 40, 5819 (2001). 4. A. Al-Zein, et al, Phys. Rev. B, 78, 134113 (2008); Phys. Rev. Lett. 105, 017601(2010). 5. Maier B et al, Phys. Rev. B, 79, 224108 (2009). 6. I. Rafalovskyi, et al, arXiv:1304.1879 (2013). 7. J. Pokorný et al, J. Adv. Phys. 5, 1550013 (2015).

Resume : A combined neutron diffuse scattering study and model analysis of the antiferroelectric crystal PbZrO3 is described. Clear evidence for disordering of oxygen octahedral tilts and Pb displacements is found in the high-temperature cubic phase. Extra scattering is found at M and R points in the Brillouin zone. A Molecular Dynamics simulation closely reproduces the diffuse scattering pattern. The arrangements of the plus (in-phase) and minus (antiphase) tilt systems are discussed according to the model. The transition between disordered and ordered local structure is discussed and compared with that seen in Zr-rich PbZr1-xTixO3.

Resume : We shall present our recent results on antiferroelectrics and antiferroelectric-based solid solutions, focusing on the atomistic origin of the wealth of extraordinary properties they exhibit when combined with ferroelectrics or magnetic materials. Starting with the model antiferroelectric lead zirconate (PbZrO3), we shall explain why the transitional phase between the high-temperature cubic phase and the room-temperature antiferroelectric phase is not always observed as well as its origin and properties. The proximity in energy with ferro- and para-electric phases will also be discussed from the behavior of PbZrO3 under high pressure, investigated from 1st-principles and experimental results. We shall also review the possibilty to engineer materials through the influence of chemical order on the onset of antiferroelectricity. After reviewing double-perovskites in general, we shall focus on lead ytterbium niobate (PYN: PbYb1/2Nb1/2O3 or Pb2YbNbO6). This material is of particular interest as its solid solution with lead zirconate (PbTiO3) exhibits all polar orders of applicative interest at room temperature, notably high-temperature high-piezoelectric constants that surpass lead zirconate titanate ones. The comparative study of the microscopic displacement patterns in the two solid solutions (PbZrO3-PbTiO3 and PYN-PbTiO3), both based on antiferroelectrics, will shed a new light on the building blocks of high-performance perovskite functional materials. The last part of the presentation will review another avenue to engineer novel properties by presenting the solid solution between an antiferroelectric material (still PbZrO3) and a magnetic perovskite (PbFe2/3W1/3O3). Short- and long-range interactions exist in both end-members. This multiferroic solid-solution exhibits not only polar and magnetic orders but also couplings of different kinds and at different length-scales, including bi-relaxors, combining the high-performances of ferroelectric relaxors with multiferroicity

Resume : Antiferroelectricity has aroused again a lot of attention, due to the new possibilities for application in devices because of its natural nano-domain structure. The prototypic antiferroelectric material is PbZrO3. However, its antiferroelectric behaviour is also in competition with a polar instability. It is, therefore, not surprising that PbZrO3 becomes ferroelectric under electric field, pressure or small ionic substitution. Small amounts of Ti induce an intermediate ferroelectric phase on cooling from the paraelectric phase, prior to the achievement of its antiferroelectric ground state. We have studied the intermediate ferroelectric states in Pb(Zr,Ti)O3, with about 5% of Ti. Dielectric, inelastic and Raman experiments show that there are several intermediate ferroelectric states in these ceramics, very dependent on the thermal pre-history of the samples. The availability of these states near room temperature depends on the delicate thermal treatment of the sample and the heating and cooling rates. A good understanding of this effect will help to develop better antiferroelectric-ferroelectric devices.

Resume : Interfaces in oxides - whether artificially introduced in superlattices or present intrinsically as ferroic domain walls - provide pathways to novel functional behaviour such as electronic transport properties or coupling between different order parameters, all at a very localised scale. Here we report on such interfaces, focusing in particular on the role of electrostatic and strain boundary conditions. We explore the complex phase diagram of epitaxial BiFeO3/LaFeO3 superlattices [1] grown on (001) SrTiO3 and (110) DyScO3 substrates, presenting three distinct regions as a function of BiFeO3 fraction, with a BiFeO3-like ferroelectric phase and a LaFeO3-like paraelectric phase at its extremities, and a complex intermediate region, as supported by first principles calculations. The intermediate region shows unusual, mixed functional behavior, most likely due to competing phases driven by substitution with a same-size central ion and the specific boundary conditions imposed by the superlattice structure, with much more diffuse transitions observed for the samples grown on SrTiO3. In epitaxial ferroelectric Pb(Zr,Ti)O3 thin films, we investigate on the effects of surface adsorbates and oxygen vacancy redistribution on the electrical conduction at domain walls. In SrTiO3-based films, domain-wall-specific conduction is present with no additional treatment [2]. In DyScO3- based films, we observe reversible switching between conducting and insulating behaviour at 180° domain walls following ultrahigh vacuum thermal annealing vs. ambient exposure, respectively [3]. 1. Rispens et al, Phys. Rev. B. 90, 104106 (2014) 2. Guyonnet et al, Adv. Mat. 23, 5377 (2011) 3. Gaponenko et al, Appl. Phys. Lett. 106, 162902 (2015)

Resume : Ferroelectrics are already firmly established as some of the most useful functional materials with a broad range of applications in non-linear optics, thermal imaging, electromechanical elements and electronics. Furthermore, there is hope that recent advances in our understanding of nanoscale ferroelectricity may enable this list to be further expanded with devices such as ferroelectric tunnel junctions, memristors and ferroelectric field effect transistors. Among the many fascinating properties of nanoscale ferroelectrics, the possibility of using them as negative capacitance elements has recently been attracting significant interest in part due to the potential benefits it may bring to the operation of field effect transistors [1]. The negative capacitance phenomenon relies on the imperfect screening of the ferroelectric polarisation, which can be engineered in a controlled manner in artificially layered systems such as ferroelectric-dielectric superlattices. We have used dielectric impedance spectroscopy to investigate the dielectric properties of such superlattices exhibiting negative capacitance effects over a broad range of temperatures. The results are interpreted using phenomenological modelling and atomistic simulations, which reveal the crucial role played by the ferroelectric domain structure in the dielectric properties of these artificially layered materials. [1] Salahuddin, Datta, Nano Letters 8, 405 (2008)

Resume : Oxides perovskites attract widespread interest due to the large scope of their functional properties, including ferroelectricity, magnetism, superconductivity, giant magnetoresistance to name a few. Among them, the members of the R3+Ni3+O3 family (1,2,3,4,5), display an interesting phase diagram. As the temperature decreases, they undergo a metal to insulator phase transition (MIT), linked to the appearance of a charge ordering/charge disproportionation and a symmetry change from Pbnm to P21/n. The only exception is LaNiO3, which adopts a R-3c phase and remains metallic (6). The charge ordering is crucial for the MIT, as it splits the eg d levels between neighboring Ni3+ sites (t2g6eg1) in two different eg occupations: t2g6eg2 (Ni2+) and t2g6eg0 (Ni4+). Moreover, this specific charge ordering is compatible with complex magnetic structures that can further give rise to improper ferroelectricity and the appearance of a sizable spontaneous polarization (3,4). Many efforts have already been devoted to the characterization of those systems. Unfortunately, standard DFT methods typically fail to reproduce their correct ground state, both in terms of symmetry and magnetic properties (3,4,5). In the present work, we report symmetry analysis and first-principles study of selected bulk R3+Ni3+O3 nickelates. Firstly, we show that our DFT approach, based on the hybrid B1WC functional (7), is able to reproduce the proper ground state symmetry and properties. This allows us to re-discuss, in bulk systems, the interplay between various structural degrees of freedom (ferroelectric, AFD rotational motions, oxygen breathing, Jahn-Teller distortions) and the orbital, charge and magnetic orderings and to identify possible new functionalities. Secondly, we discuss the influence of epitaxial strain and layering on the properties of bicolor nickelates superlattices, including the possible appearance of hybrid improper ferroelectricity. Work supported by the ARC project “TheMoTherm” and F.R.S-FNRS project “HiT4FiT”. Ph. Ghosez also acknowledges a Research Professorship of the Francqui Foundation. 1. Electric-Field Control of the Metal-Insulator Transition in Ultrathin NdNiO3 Films. R. Scherwitzl, P. Zubko, I. Gutierrez Lezama, S. Ono, A. F. Morpurgo, G. Catalan and J.-M. Triscone, Adv. Mat. 22, 5517 (2010). 2. Metal-Insulator Transition in Ultrathin LaNiO3 Films. R. Scherwitzl, S. Gariglio, M. Gabay, P. Zubko, M. Gibert and J.-M. Triscone, Phys. Rev. Lett. 106, 246403 (2011). 3. Multiferroicity in Rare-Earth Nickelates RNiO3, G. Giovannetti, S. Kumar, D. Khomskii, S. Picozzi and J. van den Brink, Phys. Rev. Lett. 103, 156401 (2009). 4. Rationalizing strain engineering effects in rare-earh nickelates. F. Y. Bruno, K. Z. Rushchanskii, S. Valencia, Y. Dumont, C. Carrétéro, E. Jacquet, R. Abrudan, S. Blügel, M. Lezaic, M. Bibes and A. Barthélémy, Phys. Rev. B 88, 195108 (2013). 5. Ab initio study of the factors affecting the ground state of rare-earth nickelates. S. Prosandeev, L. Bellaiche and J. Iñiguez, Phys. Rev. B 85, 214431 (2012). 6. Exchange Interaction in the Insulating Phase of RNiO3. J.-S. Zhou, J. B. Goodenough and B. Dabrowski, Phys. Rev. Lett. 95, 127204 (2005). 7. Hybrid exchange-correlation functional for accurate prediction of the electronic and structural properties of ferroelectric oxides. D. I. Bilc, R. Orlando, R. Shaltaf, G.-M. Rignanese, J. Iñiguez and Ph. Ghosez, Phys. Rev. B 77, 165107 (2008).

Resume : For several years, we have studied the possibility to electrically modulate the propagation of terahertz (THz) radiation using SrTiO3 (STO) strained thin films [1,2]. Indeed, the dielectric behavior of STO is controlled by the soft mode whose frequency depends on temperature and electric field. Tensile strain in STO films grown on a DyScO3 (DSO) substrate enables one to induce the ferroelectric phase transition close to room temperature [3]. DSO layers are intercalated between the STO ones to maintain the strain. With this further work, a deeper understanding of the physics of strained STO thin films has been reached [4]. A tunable structure suitable for applications is proposed. The dependence of the dielectric spectra on temperature and external electric field is described by a general model which involves a damped harmonic oscillator (soft mode) coupled to a Debye relaxation (central mode). Near the phase transition, the relaxation is silent and observable in the spectra only owing to its coupling to the polar soft mode. At lower temperatures, the soft mode hardens and exerts less influence on the THz dynamics. In contrast, the central mode becomes stronger and, under 120 K, it mainly determines the shape of the measured spectra.

Resume : Electrocaloric (EC) materials, which couple a change in temperature with an applied electric field, can address cooling applications in microelectronic systems and are subject of growing interest [1,2]. Ferroelectric oxides as PZT films appear as good candidates for this purpose [3]. However, in spite of the growing number of EC studies on pyroelectric oxides, their structure-properties relationships are still missing. In this communication, we will show the impact of the domain-orientation on the ferroelectric and EC properties of epitaxial PbZr0.52Ti0.48O3 (PZT) films. The main domain-orientation is tuned here by the thermal expansion mismatch with the substrate. SrTiO3(001) and Si(001) substrates are used to generate different orientations of the ferroelectric domains: mainly c-axis and a-axis, respectively. The EC effect is revealed to be much larger along the polarization axis and temperature changes can reach 12°C in c-oriented PZT films near room temperature under 900 kV/cm in comparison to 5°C in a-oriented, in the same conditions. This tendency is similar to what has been observed for intrinsic pyroelectric coefficients [4]. These results are of high interest for refrigeration applications, thermal energy harvesting or thermal sensing applications in integrated systems. [1] G. Sebald et al., J. Appl. Phys. 100, 124112 (2006) [2] J.F. Scott, Annu. Rev. Mater. Res. 41, 229 (2011) [3] E. Defay et al., Adv. Mat. 25, 3337 (2013) [4] R. Moalla et al., submitted