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Structural phase transition behavior of a material under temperature plays a key role in understanding its physical properties, instabilities, and design of new functions. However, there is extreme shortage of investigations on the temperature-driven phase transition behaviors of bismuth ferrite (BiFeO3). Herein, magnetic phase transitions within low-temperature regimes, ferroelectric phase transitions under high temperature, structural instability evolution, and spin-phonon coupling in 5% La-doped BiFeO3 ceramic (BFLa05) were systemically studied by employing X-ray diffraction, differential scanning calorimetry, Raman spectroscopy, and magnetic measurement, as well as the phenomenological theoretical analysis. Thanks to the outstanding stability of BFLa05 ceramic under high temperature, a complete phase transition sequence of α ↔ β ↔ γ was observed, simultaneously determining the crystalline symmetries for β and γ phases as orthorhombic Pnma and cubic Pm3m, respectively. Octahedral tilt as a typical str...

The correlation between ex situ electronic conductivity, oxygen vacancy content, flat-band potential (Efb), and the oxygen evolution reaction (OER) activity for a wide range of perovskite compositions are investigated experimentally and theoretically. It is found that all of these parameters can affect the OER activity; however, none of them alone play a crucial role in determining the electrocatalytic activity. The correlation of one single physicochemical property with the OER activity always presents deviation points, indicating that a limitation does exist for such 2-dimensional correlations. Nevertheless, these deviations can be explained considering other physicochemical properties and their correlation with the OER activity. Hence, this work aims in simultaneously linking the OER activity with several physicochemical materials properties. The concept of the OER/multidescriptor relationship represents a significant advancement in the search and design of highly active oxygen evolution catalysts, in the quest for efficient anodes in water electrolyzers.

International audience; We derive surface polarity at and near ferroelastic domain boundaries from molecular dynamics simulations based on an ionic spring model. Interatomic gradient forces lead to flexoelectricity which, in turn, generates polarity at the surface and in twin boundaries. We then derive generic properties of electron scattering spectra equivalent to those observed in low-energy electron microscopy (LEEM) and mirror electron microscopy (MEM) experiments. Negatively (positively) charged surfaces reflect (attract) incident electrons with low kinetic energy. The electron images reveal the valley and ridge surface structures near the intersection of the twin boundary and the surface. Polarity in surface layers is predicted to be visible in LEEM and MEM spectra at neutral surfaces, but much less when surfaces are charged. Inward polarity reflects electrons similar to negative surface charges, and outward polarity backscatters electrons like positive surface charges. Both the polarity in the twin boundary and the physical topography scatter electrons, consistent with experimental LEEM and MEM experiments on CaTiO3 with (001) and (111) surface terminations.

International audience; Ceramics of pure phase Yttrium (Y) doped BiFeO 3 prepared by a solid-state sintering route were characterized by X-ray diffraction, Raman spectroscopy, magnetic and electrical measurements. The results and analysis show that Y substitution greatly reduces the leakage current and enhances the multiferroic properties of BiFeO 3. Leakage conduction mechanism is shown to change from space-charge-limited conduction type in pure BiFeO 3 to a Poole–Frenkel emission behavior in Bi 0.90 Y 0.10 FeO 3. A Fowler-Nordheim tunneling mechanism in Bi 0.95 Y 0.05 FeO 3 ceramic is also evidenced under high electric fields. At the same time, enhanced magnetic properties due to Y-doping are confirmed by temperature dependent magnetometry and supported by Raman spectroscopy. An unexpected and sharp switching behavior in the magnetization under low magnetic fields observed in Bi 0.90 Y 0.10 FeO 3 ceramic, together with its improved ferroelectric property, may trigger such system for promising magneto-electric applications.

International audience; We report the growth of BaZrO3 single crystals by the optical floating zone technique and the investigation on its flux growth using BaB2O4 as a solvent. 6 mm long colorless and transparent single crystals were obtained with a mirror furnace without the need for post-treatment annealing. Its properties are determined and compared with those of two commercial crystals grown by the tri-arc Czochralski method. The chemical composition was investigated using glow discharge mass spectrometry (GDMS) and secondary ion mass spectrometry (SIMS), which indicate minor impurities of Sr, Hf, Ca and Ti, with maximal concentrations for Sr and Hf in the range of 0.3–0.5% at. The optical band gap determined by UV-visible spectroscopy is found to be ∼4.8 eV and indicates the high quality of the BaZrO3 crystals grown by the optical floating zone technique. Raman spectroscopy at ambient conditions and at low temperatures down to 4.2 K reveals a relatively sharp second-order spectrum and does not reveal any structural phase transition. Prospective high-temperature solution growth using BaB2O4 self-flux was investigated and led to 150–200 μm BaZrO3 crystals. This solvent opens the way to grow BaZrO3 at half its melting point by the flux method.

Ferroic domain walls could play an important role in microelectronics, given their nanometric size and often distinct functional properties. Until now, devices and device concepts were mostly based on mobile domain walls in ferromagnetic and ferroelectric materials. A less explored path is to make use of polar domain walls in nonpolar ferroelastic materials. Indeed, while the polar character of ferroelastic domain walls has been demonstrated, polarization control has been elusive. Here, we report evidence for the electrostatic signature of the domain-wall polarization in nonpolar calcium titanate (CaTiO3). Macroscopic mechanical resonances excited by an ac electric field are observed as a signature of a piezoelectric response caused by polar walls. On the microscopic scale, the polarization in domain walls modifies the local surface potential of the sample. Through imaging of surface potential variations, we show that the potential at the domain wall can be controlled by electron injection. This could enable devices based on nondestructive information readout of surface potential., Comment: 30 pages, 12 figures

International audience; A compressive uniaxial mechanical stress is applied on ferroelastic CaTiO3 (CTO), and a change in the domain structure is observed under a polarization microscope and a second harmonic generation (SHG) microscope. New twin walls (TWs) appear perpendicular to the original TWs under stress. The SHG microscope observations and analyses confirm that this type of stress-induced TWs is polar, similar to the original TWs, and is crystallographically prominent with monoclinic symmetry m. A quantitative estimation of this stress-induced effect reveals that CTO is hard ferroelastic in the sense that the TW movement requires a large stress. A possible application of this phenomenon is discussed.

Rare-earth ions (RE = La3+, Nd3+, and Er3+) substituted BiFeO3 (BFO) ceramics were synthesized by a conventional solid-state sintering procedure. X-ray diffraction patterns and Raman spectra confirm a rhombohedral R3c symmetry in all samples with significant distortion in FeO6 octahedron, as well as the occurrence of remarkable spin-phonon coupling and notable change in magnetic transition temperature induced by RE dopants. The enhanced magnetization was observed in all RE-doped BFO ceramics, unveiling that the spatial spin structure of BFO should be perturbed by RE dopants. Diffuse reflectance spectra show a conspicuous evolution of interband electronic transitions in RE-doped BFO ceramics. Especially, the two crystal-field d-d band transitions (6A1g→4T1 and 6A1g→4T2g) exhibit a linear red-shift behavior with the reduction in the cell volume, which is well-linked with a linear tendency of increased magnetization. On the basis of these investigations, a possible mechanism was proposed in this paper to demonstrate the correlation between the structural distortion, interband electronic transitions, and magnetic properties in RE-doped BFO ceramics.

The first successful growth of La 1/2 Na 1/2 TiO 3 (LNTO) single crystals by floating zone method is reported. This growth requires specific atmosphere conditions to avoid vaporization of Na, and a post-growth annealing under dioxygen to get pure LNTO phase. We report, for the first time, dielectric measurements on a LNTO crystal, which confirm the quantum paraelectric behavior of this material.

Temperature dependences of optical second harmonic generation (SHG) and light transmissivity are measured on a single crystal of multiferroic BiFeO3. SHG intensity exhibits a peak around the Neel temperature. The origin of this anomalous behavior could be attributed to rearrangements of ferroelectric micro-domain structures caused by spin-lattice coupling enhanced by ferroelectric order parameter.

Ferroelectric domain structures of BiFe 0 3 single crystals are observed using second harmonic generation and polarization microscopes. Anisotropy of SHG intensities is measured on (111)c and (110)c plates and analyzed by the Sapriel theory. The observed domains keep the same pattern from room temperature to above T N of 643 K without essential change.

A narrow part of Fe2O3–Bi2O3 phase diagram was re-investigated in order to elaborate single crystals of the multiferroic BiFeO3 (BFO). Centimeter-size single crystals were successfully obtained by flux method, and present a preferred growth direction. X-ray diffraction studied have highlighted that the growth direction is along the polar axis [111] r of the structure. The stability of BFO versus temperature (reversible ferroelectric transition followed by multiple irreversible decompositions) is discussed in the light of Differential Scanning Calorimetry (DSC) analysis performed between 25 and 1400°C.

L’origine du couplage de differentes proprietes physiques au sein d’un meme materiau est un sujet central de la physique de la matiere condensee et interpelle les scientifiques depuis des siecles. Nous nous interessons ici a une classe de materiaux qui possedent simultanement plusieurs proprietes dites ferroiques (ferromagnetisme, ferroelectricite et/ou ferroelasticite), dont le couplage depend de la mise en forme, de la structure cristallographique et de l’arrangement des spins magnetiques, et constitue un sujet qui combine un fascinant defi scientifique avec un grand potentiel d’applications.

Using a simple methodology based on the classical Bragg–Brentano X-ray geometry we have evidenced at room temperature a pseudo-cubic phase and a monoclinic (Pm) phase in thin films of PbSc1/2Nb1/2O3 (PSN) and (PbSc1/2Nb1/2O3)0.57–(PbTiO3)0.43 (PSN–PT) respectively, the latter compound corresponding to a morphotropic phase boundary (MPB) composition. Depending on the substrate and the thickness, different domains configuration are obtained. Competition between epitaxial and thermal stress induces a complex structural evolution with temperature which is different from that of ceramic samples. Indeed PSN films transform from a pseudo-cubic (triclinic) phase towards a cubic phase whereas in PSN–PT, a succession of transformations from a pure Pm phase toward a tetragonal (possibly non polar) phase is observed, a cubic phase never being reached up to 950 K.

The polar nature of twin boundaries (TBs) of inherently nonpolar ferroelastic CaTiO3 is examined by the confocal second harmonic generation (SHG) microscope. Three-dimensional SHG images of samples with the pseudocubic (110) and (1-11) planes confirm that both crystallographically prominent TB (W plane) and nonprominent TB (W' plane) are SHG active and consequently polar. The directions of the electric polarization PS are determined from the anisotropy of SHG intensities. In the W' plane, the SHG polar diagrams are well fitted using the point-group symmetry 2, which reveals that the PS directions parallel to the twofold axis do not lie in the W' plane but are tilted from it. In the W plane, the PS direction lies in the mirror plane of the point-group symmetry m.

Nanoparticles of strontium titanate (SrTiO3) have been synthesized in the 15 nm to 1 μm size range and studied using a combination of dielectric, Raman, x-ray, and neutron measurements. When diminishing the grain size, a strong reduction of dielectric permittivity, an enhancement of (normally forbidden) Raman polar modes and a progressive decoupling (or nonlinear coupling) between the antiferrodistortive (AFD) order parameter and the spontaneous strain, is observed. A qualitative explanation of all these effects could be achieved using the Petzelt core-shell model of SrTiO3 nanoparticles, with a core constituted of nonferroelectric AFD phase and a shell with frozen polarization. Depending on the route of synthesis, a strong increase of the AFD ferroelastic critical temperature Tc is observed. This behavior cannot be explained by considering only pure size or external strain effects, and a more complicated mechanism involving defects induced by the synthesis process should probably be considered. Interestingly, those are able to strongly affect the core grain structure, modifying thereby the macroscopic physical properties of SrTiO3 nanoparticle-based-materials.

BiFeO${}_{3}$ exhibits a complex phase-transition sequence under pressure associated with changes in octahedron tilts and displacements of Bi${}^{3+}$ and Fe${}^{3+}$ cations. Here, we investigate the local structure of Fe${}^{3+}$ as a function of pressure through absorption crystal-field spectroscopy in the 0--18 GPa range. We focus on the influence of phase transitions on the Fe${}^{3+}$ off-center displacement through the energy ($E$) and oscillator strength (${f}_{d\ensuremath{-}d}$) of the ${}^{4}$T${}_{1}$ and ${}^{4}$T${}_{2}$ Fe${}^{3+}$ (3${d}^{5}$) bands observed below the band gap (${E}_{\mathrm{gap}}$ $=$ 2.49 eV) at 1.39 and 1.92 eV, respectively, at ambient conditions. Pressure induces linear redshift of both ${}^{4}$T${}_{1}$ and ${}^{4}$T${}_{2}$ bands, consistent with the compression of the FeO${}_{6}$ octahedron under pressure. On the other hand, the transition oscillator strength (${f}_{d\ensuremath{-}d}=3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$), enabled by both the exchange mechanism and the off-center Fe${}^{3+}$ distortion, slightly increases with pressure. The absence of notable anomalies in the variation of $E$($P$) and ${f}_{d\ensuremath{-}d}$($P$) through the phase sequence from the ferroelectric rhombohedral $R$3$c$ phase to the nonpolar orthorhombic Pnma phase suggests a persisting off-center position of the Fe${}^{3+}$. While this local polarity is correlated and expected in the ferroelectric $R$3$c$ phase, its presence in the high-pressure nonpolar Pnma phase indicates the presence of local polar instabilities.

We combine a wide variety of experimental techniques to analyze two heretofore mysterious phase transitions in multiferroic bismuth ferrite at low temperature. Raman spectroscopy, resonant ultrasound spectroscopy, EPR, X-ray lattice constant measurements, conductivity and dielectric response, specific heat and pyroelectric data have been collected for two different types of samples: single crystals and, in order to maximize surface/volume ratio to enhance surface phase transition effects, BiFeO3 nanotubes were also studied. The transition at T=140.3K is shown to be a surface phase transition, with an associated sharp change in lattice parameter and charge density at the surface. Meanwhile, the 201K anomaly appears to signal the onset of glassy behaviour., submitted to PRB

International audience; The coexistence of ferroelectricity and magnetic order in epitaxial hexagonal YbFeO3 (h-YbFeO3) thin films deposited on yttria stabilized zirconia substrates is examined using an impedance analyzer, a second harmonic generation (SHG) microscope, an X-ray diffractometer, a transmission electron microscope and a superconducting quantum interference device magnetometer. The results show that h-YbFeO3 films exhibit ferroelectricity below 350K with threefold translational periodicity along the [110] direction similar to rare earth manganese oxides, and ferrimagnetism with a four-up and two-down spin configuration of Fe+3 and Yb+3 ions along the c-axis at low temperatures. In addition, a possibility of new ferroelectric phases with an electronic origin induced by the magnetic ordering is discussed based upon SHG and D-E hysteresis measurements.

We investigate the high-pressure phase transitions in BiFeO3 by single crystal and powder x-ray diffraction, as well as single crystal Raman spectroscopy. Six phase transitions are reported in the 0-60 GPa range. At low pressures, up to 15 GPa, 4 transitions are evidenced at 4, 5, 7 and 11 GPa. In this range, the crystals display large unit cells and complex domain structures, which suggests a competition between complex tilt systems and possibly off-center cation displacements. The non polar Pnma phase remains stable over a large pressure range between 11 and 38 GPa, where the distortion (tilt angles) changes only little with pressure. The two high-pressure phase transitions at 38 and 48 GPa are marked by the occurence of larger unit cells and an increase of the distorsion away from the cubic parent perovskite cell. We find no evidence for a cubic phase at high pressure, nor indications that the structure tends to become cubic. The previously reported insulator-to-metal transition at 50 GPa appears to be symmetry breaking., 11 pages, 8 figures

In this work, we investigate calcium titanate $[{\text{CaTiO}}_{3}(\text{CTO})]$ using x-ray diffraction and Raman spectroscopy up to 60 GPa and 55 GPa, respectively. Both experiments show that the orthorhombic $Pnma$ structure remains stable up to the highest pressures measured, in contradiction to ab initio predictions. A fit of the compression data with a second-order Birch-Murnaghan equation of state yields a bulk modulus ${K}_{0}$ of 181.0(6) GPa. The orthorhombic distortion is found to increase slightly with pressure, in agreement with previous experiments at lower pressures and the general rules for the evolution of perovskites under pressure. High-pressure polarized Raman spectra also enable us to clarify the Raman mode assignment of CTO and identify the modes corresponding to rigid rotation of the octahedra, $A$-cation shifts, and Ti-O bond stretching. The Raman signature is then discussed in terms of compression mechanisms.

Conditions for epitaxial growth of hexagonal YbFeO 3 (h-YbFO) using the PLD method are examined. Ferroelectricity of h-YbFO is confirmed at room temperature by dielectric, dilatometric, SHG and TEM techniques which show anomalous temperature behaviors around 350K. SQUID reveals ferrimagnetic spin configuration along the c-axis in low temperature.

International audience; Using a combination of experimental and theoretical techniques, we reveal that lead scandium niobate (PSN) nanograins exhibit a critical size of approximate to 65 nm below which the ground state is not anymore rhombohedral but rather monoclinic for chemically disordered systems. A triclinic ground state is further found for chemically ordered nanosized PSN. Such changes in symmetry are associated with a change in the polarization's direction, as well as, with an enhancement of the polarization's magnitude and a vanishing of relaxation. First-principles-based calculations point toward an internal electric field and mechanical constraints for the origins of these unexpected features.

Ferroelectric properties of ${\text{Bi}}_{1\ensuremath{-}x}{\text{Pb}}_{x}{\text{FeO}}_{3}$, through a structural approach, by structural Rietveld refinement and x-ray diffraction versus temperature have been carried out. This study allows us to draw a phase diagram for the system ${\text{Bi}}_{1\ensuremath{-}x}{\text{Pb}}_{x}{\text{FeO}}_{3}$. Although lead atom gets a lone pair and is commonly understood as having a ``ferroelectric'' character, we report in this paper that adding of lead atom in the Bi subsystem of ${\text{BiFeO}}_{3}$, destroys the long-range FE order induced by Bi lone pairs on contrary, and progressively breaks the ferroelectric ordering up to the point where the structure becomes cubic on average. The decreasing of ${T}_{C}$ with increasing Pb content also confirms that adding of Pb atoms destabilizes the ferroelectric rhombohedral order and stabilizes the cubic phase via a complex texture of mixed phases, occurring at a local scale.

We report experimental evidence for pressure instabilities in the model multiferroic BiFeO3 and namely reveal two structural phase transitions around 3 GPa and 10 GPa by using diffraction and far-infrared spectroscopy at a synchrotron source. The intermediate phase from 3 to 9 GPa crystallizes in a monoclinic space group, with octahedra tilts and small cation displacements. When the pressure is further increased the cation displacements (and thus the polar character) of BiFeO3 is suppressed above 10 GPa. The above 10 GPa observed non-polar orthorhombic Pnma structure is in agreement with recent theoretical ab-initio prediction, while the intermediate monoclinic phase was not predicted theoretically., Comment: new version, accepted for publication in Phys. Rev. B

International audience; Understanding matter by using high-pressure: a case study of functional perovskite-type oxidesThe understanding of functional ABO3 perovskite-type oxides is a very active research area at the interfacebetween solid state chemistry and physics. The central idea of this study is to use the external parameterhigh-pressure to elucidate the chemical/physical mechanisms that are at the origin of the remarkablephysical properties. Our study of various perovskites with outstanding properties (relaxor ferroelectrics,multiferroics etc.) reveals that the parameter pressure is indeed a very useful tool to extract essentialstructural information which in turn provides a deeper understanding of the underlying mechanisms, andsometimes even allows discovering entirely new physical phenomena.; La compréhension d’oxydes fonctionnels de type pérovskite ABO3 est un domaine de recherche très vaste et actif, à l’interface entre la chimie et la physique de l’état solide. L’idée centrale de cette étude est d’élucider les mécanismes physico-chimiques régissant les propriétés de ces oxydes pérovskites en utilisant le paramètre externe haute pression afin de modifier l’équilibre entre les interactions (électrostatique, élastique, électronique…) à courte et à longue distances.À travers l’étude de matériaux à propriétés exceptionnelles tels que les ferroélectriques relaxeurs ou les multiferroïques ou des matériaux plus communs tels que les ferroélectriques classiques, la pression se révèle être un outil d’analyse original qui permet de scruter la matière à toutes les échelles afin d’en extraire des informations structurales et, dans certains cas, d’accéder à des phénomènes physiques inattendus.

International audience; An effective Hamiltonian scheme is developed to study finite-temperature properties of multiferroic BiFeO3. This approach reproduces very well (i)the symmetry of the ground state, (ii)the Néel and Curie temperatures, and (iii)the intrinsic magnetoelectric coefficients (that are very weak). This scheme also predicts (a)an overlooked phase above TC 1100K that is associated with antiferrodistortive motions, as consistent with our additional x-ray diffractions, (b)improperlike dielectric features above TC, and (c)that the ferroelectric transition is of first order with no group-subgroup relation between the paraelectric and polar phases. © 2007 The American Physical Society.

Regularly twinned domain structures are observed by scanning piezoforce microscopy on single crystalline grains of BiFeO3 ceramics being grown by a special low temperature sintering process. The domains are considerably larger than those observed in thin films. Their spontaneous polarization comes close to that predicted theoretically and overcomes restrictions hitherto being set to bulk single crystals. The observed ferroelastic twin domain structure resembles that of classic T domains in rhombohedrally distorted NiO, but is additionally superimposed by ferroelectric twin domain patterns.

International audience; The perovskite Bismuth ferrite BiFeO3 (BFO) is considered to be a model multiferroic and offers a rare multiferroelectric robustness since it presents a coexistence of ferroelectric and antiferromagnetic order up to unusually high temperatures. Perovskite-type materials are known for their common structural instabilities which can be driven by diverse external parameters like temperature, pressure, stress etc. Such instabilities and the associated structural distortions are often very subtle and difficult to detect by techniques probing the average structure such as X-ray diffraction. Here we present an investigation of the BFO phonon spectrum by the local probe Raman spectroscopy as a function of polarization, temperature and pressure. We review a recent temperature-dependent Raman investigation which illustrates a first-order structural phase transition at the ferroelectric Curie temperature. Our temperature dependence results further indicate a phonon-anomaly around the magnetic Néel temperature, which is discussed in the light of multiferroicity. We will further illustrate that BFO presents important pressure-induced structural instabilities and we discuss the role of such instabilities for the understanding of strained thin film BFO which show distinct properties compared to the bulk.

We have investigated the whole phase diagram of ${\mathrm{PbSc}}_{1∕2}{\mathrm{Nb}}_{1∕2}{\mathrm{O}}_{3}\ensuremath{-}{\mathrm{PbTiO}}_{3}$ (PSN-PT) solid solution by Raman scattering and compared the results to our recent x-ray and theoretical study on this system [R. Haumont et al., Phys. Rev. B 71, 104106 (2005)]. Changes in the vibration bands associated to the sequence of phase transitions $R3m\text{\ensuremath{-}}Cm\text{\ensuremath{-}}Pm\text{\ensuremath{-}}P4mm$ by doping PSN with ${\mathrm{Ti}}^{4+}$ were evidenced, especially between the two monoclinic phases. The detailed analysis also pointed out that the monoclinic order probably already exists into the rhombohedral and the tetragonal phases on a local scale which makes the morphotropic phase region (MPR) more extended than it was reported. Moreover, besides the polar states this Raman analysis evidences the strong chemical inhomogeneities existing on a nanometer scale in such relaxor-based system such as the Ti-rich and $\mathrm{Sc}∕\mathrm{Nb}$-rich regions.

We have recently evidenced the existence of a complex path of polarisation rotation via two monoclinic phases in the giant-piezoelectric materials PSN–PT that is similar to the PMN–PT system. In this article we have presented the state of knowledge on the PSN–PT system and have given new results showing the close connection between the local inhomogeneous chemical order of Sc/Nb/Ti cations and the stability of different ferroelectric phases. The notion of morphotropic phase boundaries is discussed in relation with the gradual appearance of long-range polar order with increasing Ti content in PSN–PT. Comparison with the films and nanopowders/ceramics has also been made.

International audience; In this paper, we show that a pure single phase by doping Fe-site of BiFeO3 (BFO) using tetravalent Zr4+ ions can be achieved by introducing cation (Bi3+) vacancies. The structural analysis reveals that the ferroelectric nature of BFO should be weakly affected by 10% of Zr4+ doping as the c/a ratio and the Curie temperature T-C remain roughly unchanged compared to that of pure BFO. In contrast, the magnetic properties are affected as a weak ferromagnetism and a change of Neel temperature T-N are observed. Beyond the double-exchange interactions arising from the creation of Fe2+, we propose another simple model inducing a local ferromagnetic coupling rather than an antiferromagnetic which considers the replacement of the magnetically active Fe3+, time to time, by a nonactive Zr4+.

Anisotropy of the optical second harmonic generation (SHG) in a multiferroic BiFeO3 single crystal is observed by an SHG microscope. Polarization dependences of SH intensities are measured in several polarization configurations of the incident and SH waves to determine the magnitude and sign of polar i-type SHG tensor components. To obtain proper values of these SHG tensors, the effects of the interference term and wave number mismatch are taken into account in these analyses. d24 component is 15 times larger than d32 and this result is essentially different from the thin film case.

Highly oriented Pb(Sc1/2Nb1/2)O3 (PSN) and Pb(Sc1/2Nb1/2)O3/43%PbTiO3 (PSN/43%PT) thin films were fabricated on MgO and SrTiO3 (001) substrates by pulsed laser deposition (PLD) technique. La1/2Sr1/2CoO3 was deposited on the substrate as a bottom electrode layer for dielectric measurements. The orientation of the films was checked by X-ray diffraction method, and is found to be well oriented along the c-axis for PSN and PSN/PT. Film thickness was determined by the Laue oscillation of X-ray diffraction profile. This enables us to determine the number of pulses for stacking one layer of PSN and PSN/PT thin films. The temperature dependences of lattice constants of PSN and PSN/PT thin films were determined. They exhibit small but clear changes at almost the same temperature as that of the ferroelectric transition points of PSN and PSN/PT ceramics. The surface roughness of these films was observed using atomic force and scanning electron microscopes. Temperature change of dielectric constant ε and D-E hysteresis loop were determined for PSN and the real part of ε shows 850.

The bulk electronic structure, surface composition, conductivity, and electrochemical activity toward the oxygen evolution reaction for the La1–xSrxCoO3 perovskite series (with x = 0, 0.2, 0.4, 0.6, 0.8, 1) are investigated experimentally and theoretically. It is found that Sr substitutions have the effect of straightening the octahedral cage, aligning atoms along the Co–O–Co axis, and increasing the average oxidation state of the Co cations. As a consequence, both the ex situ electronic conductivity as well as the activity toward the oxygen evolution reaction are considerably improved. According to density-functional theory calculations, the alignment of the Co–O–Co bonds and the oxidation of the Co cations enhance the overlap between the occupied O 2p valence bands and the unoccupied Co 3d conduction bands, rationalizing the improvement of the conductivity as a function of the Sr fraction. Additionally, a study of the surface properties as a function of the Sr fraction, carried out by X-ray photoelectro...