Search

The role of the structural distortions on the metal to insulator transition (MIT) of NdNiO3 is still a controversial topic. NdNiO3 ceramic, 260 nm and 110 nm thin films deposited onto (001)-oriented LaAlO3 substrates were studied to unravel the effect of structural symmetry breaking on the electronic phase transition. Resistivity measurements evidence the coexistence of metallic and insulating states below the TMI, while magnetization measurements confirm the expected TMI = TN for all samples. The structural symmetry was investigated across the MIT, through Raman scattering spectroscopy, with the orthorhombic Pnma phase being found at high temperatures, but the emergence of the monoclinic P21/n phase only occurring well below TMI, which is an unexpected experimental result since the structural transition has been understood as a necessary trigger for the MIT. These experimental results, which are noticeably independent of the sample type/quality, imply a temperature difference between the structural and electronic/magnetic ordering, thus contrasting with current theoretical models.

In this paper, we revisit the high-pressure behavior of BaZrO3 by a combination of first-principle calculations, Raman spectroscopy, and x-ray diffraction under high pressure. We confirm experimentally the cubic-to-tetragonal transition at 10 GPa and find no evidence for any other phase transition up to 45 GPa, the highest pressures investigated, at variance with past reports. We re-investigate phase stability with density functional theory considering not only the known tetragonal (I4/mcm) phase but also other potential antiferrodistortive candidates. This shows that the tetragonal phase becomes progressively more stable upon increasing pressure as compared to phases with more complex tilt systems. The possibility for a second transition to another tilted phase at higher pressures, and in particular to the very common orthorhombic Pnma structure, is therefore ruled out.

Thin films of the solid solution Nd 1 − x La x NiO3 are grown in order to study the expected 0 K phase transitions at a specific composition. We experimentally map out the structural, electronic and magnetic properties as a function of x and a discontinuous, possibly first order, insulator–metal transition is observed at low temperature when x = 0.2. Raman spectroscopy and scanning transmission electron microscopy show that this is not associated with a correspondingly discontinuous global structural change. On the other hand, results from density functional theory (DFT) and combined DFT and dynamical mean field theory calculations produce a 0 K first order transition at around this composition. We further estimate the temperature-dependence of the transition from thermodynamic considerations and find that a discontinuous insulator–metal transition can be reproduced theoretically and implies a narrow insulator–metal phase coexistence with x. Finally, muon spin rotation (µSR) measurements suggest that there are non-static magnetic moments in the system that may be understood in the context of the first order nature of the 0 K transition and its associated phase coexistence regime.

Helium implantation in epitaxial thin films is a way to control the out-of-plane deformation independently from the in-plane strain controlled by epitaxy. In particular, implantation by means of a helium microscope allows for local implantation and patterning down to the nanometer resolution, which is of interest for device applications. We present here a study of bismuth ferrite (BiFeO3) films where strain was patterned locally by helium implantation. Our combined Raman, XRD and TEM study shows that the implantation causes an elongation of the BiFeO3 unit cell and ultimately a transition towards the so-called super-tetragonal polymorph via states with mixed phases. In addition, TEM reveals the onset of amorphization at a threshold dose that does not seem to impede the overall increase in tetragonality. The phase transition from the R-like to T-like BiFeO3 appears as first-order in character, with regions of phase coexistence and abrupt changes in lattice parameters.

Ferroelectric and ferroelastic domain walls are 2D topological defects with thicknesses approaching the unit cell level. When this spatial confinement is combined with observations of emergent functional properties, such as polarity in non-polar systems or electrical conductivity in otherwise insulating materials, it becomes clear that domain walls represent new and exciting objects in matter. In this Review, we discuss the exotic polarization profiles that can arise at domain walls with multiple order parameters and the different mechanisms that lead to domain-wall polarity in non-polar ferroelastic materials. The emergence of energetically degenerate variants of the domain walls themselves suggests the existence of interesting quasi-1D topological defects within such walls. We also provide an overview of the general notions that have been postulated as fundamental mechanisms responsible for domain-wall conduction in ferroelectrics. We then discuss the prospect of combining domain walls with transition regions observed at phase boundaries, homo- and heterointerfaces, and other quasi-2D objects, enabling emergent properties beyond those available in today’s topological systems. Ferroelectric and ferroelastic domain walls are 2D topological defects with thicknesses approaching the unit cell level and emergent functional properties. This Review discusses the exotic polarization profiles that arise at domain walls and the fundamental mechanisms responsible for domain-wall conduction.

Bismuth vanadate (BiVO4) has recently been under focus for its potential use in photocatalysis thanks to its well-suited absorption edge in the visible light range. Here, we characterize the optical absorption of a BiVO4 single crystal as a function of temperature and polarization direction by reflectance and transmittance spectroscopy. The optical bandgap is found to be very sensitive to the temperature, and to the tetragonal-to-monoclinic ferroelastic transition at 523 K. The anisotropy, as measured by the difference in the absorption edge for the light polarized parallel and perpendicular to the principal axis, is reduced from 0.2 eV in the high-temperature tetragonal phase to 0.1 eV at ambient temperature. We show that this evolution is dominantly controlled by the ferroelastic shear strain. These findings provide a route for further optimization of bismuth vanadate-based light absorbers in photocatalytic devices., APL Materials, 8 (8), ISSN:2166-532X

Collective rotations and tilts of oxygen polyhedra play a crucial role in the physical properties of complex oxides such as magnetism and conductivity. Such rotations can be tuned by preparing thin films in which dimensionality, strain, and interface effects come into play. However, little is known of the tilt and rotational distortions in films a few unit cells thick including the question of if coherent tilt patterns survive at all in this ultrathin limit. Here, a series of films of perovskite LaNiO3 is studied and it is shown that the phonon mode related to oxygen octahedral tilts can be followed by Raman spectroscopy down to a film thickness of three pseudocubic perovskite unit cells (∼1.2 nm). To push the limits of resolution to the ultrathin regime, a statistical analysis method is introduced to separate the Raman signals of the film and substrate. Most interestingly, these analyses reveal a pronounced hardening of the tilt vibrational mode in the thinnest films. A comparison between the experimental results, first principles simulations of the atomic structure, and the standing wave model, which accounts for size effects on the phononic properties, reveals that in the ultrathin regime, the Raman spectra are a hybrid entity of both the bulk and surface phononic behavior. These results showcase Raman spectroscopy as a powerful tool to probe the behavior of perovskite films down to the ultrathin limit., APL Materials, 8 (6), ISSN:2166-532X

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.

Professor Wojciech Kuczynski, a Research Staff Member at the Institute of Molecular Physics of the Polish Academy of Sciences in Poznan, Poland, passed away on 5 December 2020.Professor Kuczynski w...

${\mathrm{BaZrO}}_{3}$ is a perovskite that remains in the simple cubic phase at all temperatures, hence with no first-order Raman-active phonon mode allowed by symmetry. Yet, it exhibits an intense Raman spectrum with sharp and well-defined features. Here, we report the evolution of the Raman spectrum of ${\mathrm{BaZrO}}_{3}$ single crystals in a broad temperature range (4--1200 K) and discuss its origin with the support of detailed first-principle calculations of the lattice dynamics. Phonon calculations are performed not only for the cubic phase of ${\mathrm{BaZrO}}_{3}$, but also for the low-symmetry phases with octahedra tilts that have been suspected to exist at the nanoscale. We show that the Raman spectrum shows no direct evidence for these nanodomains, but can instead be explained by classical second-order Raman scattering. We provide an assignment of the dominant features to phonon mode combinations. In particular, we show that the high frequency range of the spectrum is dominated by overtones and shows an image of the phonon density of states corresponding to the stretching modes of the oxygen octahedra.

We studied the light-induced effects in BiFeO$_3$ single crystals as a function of temperature by means of optical spectroscopy. Here we report the observation of several light-induced absorption features, which are discussed in terms of the photostriction effect and are interpreted in terms of excitons. The temperature dependence of their energy position suggests a possible coupling between the excitons and the lattice vibrations. Moreover, there are hints for anomalies in the temperature evolution of the excitonic features, which might be related to the temperature-induced magnetic phase transitions in BiFeO$_3$. Our findings suggest a coupling between light-induced excitons and the lattice and spin degrees of freedom, which might be relevant for the observed ultrafast photostriction effect in multiferroic BiFeO$_3$., Comment: 7 pages, 6 figures

Bulk photovoltaic current in ferroelectric lithium niobate increases by 75% under a compressive mechanical stress of only 10 MPa., Photovoltaic phenomena are widely exploited not only for primary energy generation but also in photocatalytic, photoelectrochemistry, or optoelectronic applications. In contrast to the interface-based photovoltaic effect of semiconductors, the anomalous or bulk photovoltaic effect in ferroelectrics is not bound by the Shockley-Queisser limit and, thus, can potentially reach high efficiencies. Here, we observe in the example of an Fe-doped LiNbO3 bulk single crystal the existence of a purely intrinsic “piezophotovoltaic” effect that leads to a linear increase in photovoltaic current density. The increase reaches 75% under a low uniaxial compressive stress of 10 MPa, corresponding to a strain of only 0.005%. The physical origin and symmetry properties of the effect are investigated, and its potential for strain-tuned efficiency increase in nonconventional photovoltaic materials is presented.

A strong coupling of the lattice to functional properties is observed in many transition metal oxide systems, such as the ABO(3) perovskites. In the quest for tailor-made materials, it is essential to be able to control the structural properties of the compound(s) of interest. Here, thin film solid solutions that combine NdNiO3 and LaNiO3, two materials with the perovskite structure but distinct space groups, are analyzed. Raman spectroscopy and scanning transmission electron microscopy are combined in a synergistic approach to fully determine the mechanism of the structural crossover with chemical composition. It is found that the symmetry transition is achieved by phase coexistence in a way that depends on the substrate selected. These results carry implications for analog-tuning of physical properties in future functional materials based on these compounds.

Ferroelectrics carry a switchable spontaneous electric polarization. This polarization is usually coupled to strain, making ferroelectrics good piezoelectrics. When coupled to magnetism, they become so-called multiferroic systems, a field that has been widely investigated since 2003. While ferroelectrics are birefringent and non-linear optically transparent materials, the coupling of polarization with optical properties has received, since 2009, renewed attention, triggered notably by low-bandgap ferroelectrics suitable for sunlight spectrum absorption and original photovoltaic effects. Consequently, power conversion efficiencies up to 8.1% were recently achieved and values of 19.5% were predicted, making photoferroelectrics promising photovoltaic alternatives. This article aims at providing an up-to-date review on this emerging and rapidly progressing field by highlighting several important issues and parameters, such as the role of domain walls, ways to tune the bandgap, consequences arising from the polarization switchability, and the role of defects and contact electrodes, as well as the downscaling effects. Beyond photovoltaicity, other polarization-related processes are also described, like light-induced deformation (photostriction) or light-assisted chemical reaction (photostriction). It is hoped that this overview will encourage further avenues to be explored and challenged and, as a byproduct, will inspire other research communities in material science, e.g., so-called hybrid halide perovskites.

Professor Ihor Stasyuk, a prominent scientist and lecturer passed away on 16 September 2019. He significantly contributed with remarkable results to the quantum many-body physics and to the theoret...

This work reports on the pressure dependence of the octahedral tilts and mean Fe-O bond lengths in $R\mathrm{Fe}{\mathrm{O}}_{3}$ ($R=\mathrm{Nd}$, Sm, Eu, Gd, Tb, and Dy), determined through synchrotron x-ray diffraction and Raman scattering, and their role on the pressure-induced phase transition displayed by all of these compounds. For larger rare-earth cations (Nd-Sm), both anti- and in-phase octahedral tilting decrease as pressure increases, whereas the reverse behavior is observed for smaller ones (Gd-Dy). $\mathrm{EuFe}{\mathrm{O}}_{3}$ stands at the borderline, with nearly pressure-independent tilt angles. For the compounds where the tilts increase with pressure, the $\mathrm{Fe}{\mathrm{O}}_{6}$ octahedra are compressed at lower rates than for those ones exhibiting opposite pressure tilt dependence. The crossover between the two opposite pressure behaviors is discussed in relation to the general rules proposed from different theoretical approaches. The similarity of the pressure-induced isostructural insulator-to-metal phase transition, observed in the whole series, points out that the tilts play a minor role in its driving mechanisms. A clear relationship between octahedral compressibility and critical pressure is ascertained.

The pressure dependence of light-induced effects in single-crystalline BiFeO$_3$ is studied by optical spectroscopy. At low pressures, we observe three light-induced absorption features with energies just below the two crystal field excitations and the absorption onset, respectively. These absorption features were previously ascribed to excitons, possibly connected with the ultra-fast photostriction effect in BiFeO$_3$. The pressure-induced redshift of the absorption features follows the pressure dependence of the corresponding crystal field excitations and absorption onset, suggesting the link between them. Above the structural phase transition at $P_{\mathrm{c1}}\approx{}3.5$ GPa the three absorption features disappear, suggesting their connection to the polar phase in BiFeO$_3$. The pressure-induced disappearance of the photo-induced features is irreversible upon pressure release., Comment: 7 pages, 7 figures; accepted for publication in the Journal of Applied Physics

This work reports the changes in structure and lattice dynamics induced by substituting the Jahn-Teller-active $\mathrm{M}{\mathrm{n}}^{3+}$ ion by the Jahn-Teller-inactive $\mathrm{F}{\mathrm{e}}^{3+}$ in $\mathrm{TbM}{\mathrm{n}}_{1\text{\ensuremath{-}}x}\mathrm{F}{\mathrm{e}}_{x}{\mathrm{O}}_{3}$ over the full composition range. The structural analysis reveals that the amplitude of the cooperative Jahn-Teller distortion decreases linearly from $x=0$ (pure $\mathrm{TbMn}{\mathrm{O}}_{3})$ to $x=0.5$, where it is completely suppressed. We then correlate this evolution with the behavior of the Raman modes across the solid solution. In particular, we show that the Raman modes associated with the rotation of octahedra, whose wave number is commonly considered to scale linearly with the tilt angles in orthorhombic Pnma perovskites, are also sensitive to the amplitude of the Jahn-Teller distortion.

We present a systematic study of the structure and lattice dynamics at room temperature, and the phonon behavior at low temperatures in orthorhombic Gd{sub 1−x}Y{sub x}MnO{sub 3} manganites, with 0≤x≤0.4. Powder x-ray diffraction and Raman scattering at ambient conditions reveal that the Jahn–Teller distortion is composition-independent, while the Mn–O1–Mn bond angle increases by about 0.5° from x=0 to x=0.4. The spin–phonon coupling was ascertained from the relation between the renormalized wavenumber of the in-plane O2 stretching mode, associated with the spin–spin correlation function. The balance between competitive magnetic interactions is discussed based on the temperature dependence of the phonon frequency renormalization. - Graphical abstract: Wavenumber of the in-plane O2 stretching mode as a function of the spin–spin correlation function, for GdMnO{sub 3} (a), and Gd{sub 0.8}Y{sub 0.2}MnO{sub 3} (b). - Highlights: • Crucial role of structural distortions induced by ion substitution. • Tuning of magnetic interactions through ion substitution. • Facing spin–phonon coupling and competitive exchange magnetic interactions. • Scaling of spin–phonon and spin–spin mechanisms.

The domain structure of uniaxial ferroelectric lithium niobate single crystals is investigated using Raman spectroscopy mapping. The influence of doping with magnesium and poling at room temperature is studied by analysing frequency shifts at domain walls and their variations with dopant concentration and annealing conditions. It is shown that defects are stabilized at domain walls and that changes in the defect structures with Mg concentration can be probed by the shift of Raman modes. We show that the signatures of polar defects in the bulk and at the domain walls differ., 10 pages, 3 figures

Ferroic domain walls are currently investigated by several state-of-the art techniques in order to get a better understanding of their distinct, functional properties. Here, Principal Component Analysis (PCA) of Raman maps is used to study ferroelectric domain walls (DWs) in LiNbO3 and ferroelastic DWs in NdGaO3. It is shown that PCA allows to quickly and reliably identify small Raman peak variations at ferroelectric DWs and that the value of a peak shift can be deduced - accurately and without a-priori - from a first order Taylor expansion of the spectra. The ability of PCA to separate the contribution of ferroelastic domains and DWs to Raman spectra is emphasized. More generally, our results provide a novel route for the statistical analysis of any property mapped across a DW., 17 pages, 8 figures

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

Hybrid nanocomposites based on ferrimagnetic (FiM) Fe3O4 and magnetoelectric antiferromagnetic (AFM) Cr2O3 nanocrystals were synthesized to offer a particular three-dimensional (3D) interface between the two oxides. This interface favours an intermixing process (demonstrated by combining Raman spectroscopy and magnetization measurements) that determines the final magnetic behavior.

A new mixed rare-earth orthochromite series, LaxSm1-xCrO3, prepared through single-step hydrothermal synthesis is reported. Solid solutions (x = 0, 0.25, 0.5, 0.625, 0.75, 0.875, and 1.0) were prepared by the hydrothermal treatment of amorphous mixed-metal hydroxides at 370 °C for 48 h. Transmission electron microscopy (TEM) reveals the formation of highly crystalline particles with dendritic-like morphologies. Rietveld refinements against high-resolution powder X-ray diffraction (PXRD) data show that the distorted perovskite structures are described by the orthorhombic space group Pnma over the full composition range. Unit cell volumes and Cr-O-Cr bond angles decrease monotonically with increasing samarium content, consistent with the presence of the smaller lanthanide in the structure. Raman spectroscopy confirms the formation of solid solutions, the degree of their structural distortion. With the aid of shell-model calculations the complex mixing of Raman modes below 250 cm(-1) is clarified. Magnetometry as a function of temperature reveals the onset of low-temperature antiferromagnetic ordering of Cr(3+) spins with weak ferromagnetic component at Néel temperatures (TN) that scale linearly with unit cell volume and structural distortion. Coupling effects between Cr(3+) and Sm(3+) ions are examined with enhanced susceptibilities below TN due to polarization of Sm(3+) moments. At low temperatures the Cr(3+) sublattice is shown to undergo a second-order spin reorientation observed as a rapid decrease of susceptibility.

Standard Landau theory coupled to infinitesimal strain allows a concise description of the temperature-driven ferroelectric tetragonal-to-cubic phase transition in ${\mathrm{PbTiO}}_{3}$ at ambient pressure. Unfortunately, it fails to cover its high-pressure counterpart at ambient temperature. For example, the experimental transition pressure is vastly underestimated, and neither the change from first to second order with increasing pressure nor the unusual pressure dependence of the tetragonal unit cell parameters observed in experiment are reproduced. Here we demonstrate that a combination of density functional theory and a recently constructed finite-strain extension of Landau theory provides a natural mechanism for resolving these discrepancies between theory and experiment. Our approach also allows us to determine the full tetragonal-cubic phase boundary in the $(P,T)$ plane including an estimate of the tricritical point. We show that a careful analysis of the thermal elastic baseline is an essential ingredient to the success of this theory.

The rotation of octahedra (octahedral tilting) is common in ABO3 perovskites and relevant to many physical phenomena, ranging from electronic and magnetic properties, metal-insulator transitions to improper ferroelectricity. Hydrostatic pressure is an efficient way to tune and control octahedral tiltings. However, the pressure behavior of such tiltings can dramatically differ from one material to another, with the origins of such differences remaining controversial. In this work, we discover several new mechanisms and formulate a set of simple rules that allow to understand how pressure affects oxygen octahedral tiltings, via the use and analysis of first-principles results for a variety of compounds. Besides the known A-O interactions, we reveal that the interactions between specific B-ions and oxygen ions contribute to the tilting instability. We explain the previously reported trend that the derivative of the oxygen octahedral tilting with respect to pressure (dR/dP) usually decreases with both the tolerance factor and the ionization state of the A-ion, by illustrating the key role of A-O interactions and their change under pressure. Furthermore, three new mechanisms/rules are discovered. We further predict that the polarization associated with the so-called hybrid improper ferroelectricity could be manipulated by hydrostatic pressure, by indirectly controlling the amplitude of octahedral rotations., Comment: Submitted to Phys. Rev

We report a Raman scattering study of six rare-earth orthoferrites $R{\mathrm{FeO}}_{3}$, with $R$ = La, Sm, Eu, Gd, Tb, Dy. The use of extensive polarized Raman scattering of ${\mathrm{SmFeO}}_{3}$ and first-principles calculations enable the assignment of the observed phonon modes to vibrational symmetries and atomic displacements. The assignment of the spectra and their comparison throughout the whole series allow correlating the phonon modes with the orthorhombic structural distortions of $R{\mathrm{FeO}}_{3}$ perovskites. In particular, the positions of two specific $A{}_{g}$ modes scale linearly with the two ${\mathrm{FeO}}_{6}$ octahedra tilt angles, allowing the distortion to be tracked throughout the series. At variance with literature, we find that the two octahedra tilt angles scale differently with the vibration frequencies of their respective $A{}_{g}$ modes. This behavior, as well as the general relations between the tilt angles, the frequencies of the associated modes, and the ionic radii are rationalized in a simple Landau model. The reported Raman spectra and associated phonon-mode assignment provide reference data for structural investigations of the whole series of orthoferrites.

The ability to generate efficient giga–terahertz coherent acoustic phonons with femtosecond laser makes acousto-optics a promising candidate for ultrafast light processing, which faces electronic device limits intrinsic to complementary metal oxide semiconductor technology. Modern acousto-optic devices, including optical mode conversion process between ordinary and extraordinary light waves (and vice versa), remain limited to the megahertz range. Here, using coherent acoustic waves generated at tens of gigahertz frequency by a femtosecond laser pulse, we reveal the mode conversion process and show its efficiency in ferroelectric materials such as BiFeO3 and LiNbO3. Further to the experimental evidence, we provide a complete theoretical support to this all-optical ultrafast mechanism mediated by acousto-optic interaction. By allowing the manipulation of light polarization with gigahertz coherent acoustic phonons, our results provide a novel route for the development of next-generation photonic-based devices and highlight new capabilities in using ferroelectrics in modern photonics., Electrically driven acousto-optic light modulators are limited to frequencies of a few hundred megahertz and are typically no smaller than a few micrometres. Here, the authors demonstrate gigahertz acousto-optic conversion of light polarization in a region of a few nanometres using pulsed laser stimulation of a ferroelectric.

A marked conductivity enhancement is reported in 6-11 unit cell LaNiO

The understanding of domain structures, specifically domain walls, currently attracts a significant attention in the field of (multi)-ferroic materials. In this article, we analyze contrast formation in full field electron microscopy applied to domains and domain walls in the uniaxial ferroelectric lithium niobate, which presents a large 3.8 eV band gap and for which conductive domain walls have been reported. We show that the transition from Mirror Electron Microscopy (MEM – electrons reflected) to Low Energy Electron Microscopy (LEEM – electrons backscattered) gives rise to a robust contrast between domains with upwards (Pup) and downwards (Pdown) polarization, and provides a measure of the difference in surface potential between the domains. We demonstrate that out-of-focus conditions of imaging produce contrast inversion, due to image distortion induced by charged surfaces, and also carry information on the polarization direction in the domains. Finally, we show that the intensity profile at domain walls provides experimental evidence for a local stray, lateral electric field.

Ferroelectrics carry a switchable spontaneous electric polarization. This polarization is usually coupled to strain, making ferroelectrics good piezoelectrics. When coupled to magnetism, they become so-called multiferroic systems, a field that has been widely investigated since 2003. While ferroelectrics are birefringent and non-linear optically transparent materials, the coupling of polarization with optical properties has received, since 2009, renewed attention, triggered notably by low-bandgap ferroelectrics suitable for sunlight spectrum absorption and original photovoltaic effects. Consequently, power conversion efficiencies up to 8.1% were recently achieved and values of 19.5% were predicted, making photoferroelectrics promising photovoltaic alternatives. This article aims at providing an up-to-date review on this emerging and rapidly progressing field by highlighting several important issues and parameters, such as the role of domain walls, ways to tune the bandgap, consequences arising from the polarization switchability, and the role of defects and contact electrodes, as well as the downscaling effects. Beyond photovoltaicity, other polarization-related processes are also described, like light-induced deformation (photostriction) or light-assisted chemical reaction (photostriction). It is hoped that this overview will encourage further avenues to be explored and challenged and, as a byproduct, will inspire other research communities in material science, e.g., so-called hybrid halide perovskites.

International audience; The aim of the present paper is to review and briefly discuss recent experimental results on the effect of high pressure (p > 1.5 GPa) on relaxor ferroelectrics, exemplified by Na1/2Bi1/2TiO3 and PbMg1/3Nb2/3O3. The main result is that relaxor ferroelectrics present important polar and structural pressure-instabilities, which lead to both phase transitions of the average structure and considerable changes of the local structure. It is in particular the observation of a pressure-induced suppression of the relaxor-characteristic X-ray diffuse scattering that points to important changes in the local order. The complementary use of Raman spectroscopy, X-ray diffraction and X-ray diffuse scattering allows the proposition of a mechanism for the observed changes.

In recent years a number of imaging techniques to determine the optical properties of materials, either in reflection or in transmission, have been developed. Here the use of an imaging version of the so-called rotating-polarizer method in the study of phase transformations in crystals is demonstrated. This method creates false-coloured images representing the light transmissionI0, the extinction angle φ (orientationof the optical indicatrix) and |sin δ|, a function of the retardation resulting from the birefringence (and a measure of themagnitudeof optical anisotropy). When combined with a computer-controlled heating stage, this method provides an opportunity to createseparatemoving images oforientationandmagnitudeof optical anisotropy, showing the dynamics of twinning and domain-wall behaviour during temperature changes. It is believed that this is the first time that quantitative imaging of changes in birefringence has been used in this way to describe phase transitions. Two-phase transitions in a crystal of Na0.5Bi0.5TiO3(NBT) are presented as examples of the use of the system.

Polar tweed was discovered in mechanically stressed LaAlO3. Local patches of strained material (diameter ca. 5 μm) form interwoven patterns seen in birefringence images, Piezo-Force Microscopy (PFM) and Resonant Piezoelectric Spectroscopy (RPS). PFM and RPS observations prove unequivocally that electrical polarity exists inside the tweed patterns of LaAlO3. The local piezoelectric effect varies greatly within the tweed patterns and reaches magnitudes similar to quartz. The patterns were mapped by the shift of the Eg soft-mode frequency by Raman spectroscopy.

Knowledge of the electronic band structure of multiferroic oxides, crucial for the understanding and tuning of photo-induced effects, remains very limited even in the model and thoroughly studied BiFeO3. Here, we investigate the electronic band structure of BiFeO3 using Raman scattering with twelve different excitation wavelengths ranging from the blue to the near infrared. We show that resonant Raman signatures can be assigned to direct and indirect electronic transitions, as well as in-gap electronic levels, most likely associated to oxygen vacancies. Their temperature evolution establishes that the remarkable and intriguing variation of the optical band-gap can be related to the shrinking of an indirect electronic band-gap, while the energies for direct electronic transitions remains nearly temperature independent., Comment: 5 pages, 4 figures

A comparative analysis of Raman spectra from poly-domain PbTiO3 thin films with poly-domain and single-domain PbTiO3 crystals is presented. Raman spectroscopy reveals that the profiles of A1-symmetry modes can be significantly modified by the existence of the domain structure. The possible origins of these complex profiles of the A1(LO) and A1(TO) modes are discussed. It is emphasized that analysis of stresses, domain structure and phase transitions in the PbTiO3 thin films has to be based on the E(TO) modes presenting profiles that are not affected by the domain structure.

Dense ceramics of (1−x)BaTiO3−xLaYO3 (LBTY) (0≤x≤0.50) have been fabricated by the conventional solid-state route. Phase purity and crystal structure of LBTY ceramics were investigated using a combination of X-ray diffraction (XRD), electron diffraction (ED), and Raman spectroscopy. XRD analysis shows the tetragonal distortion of undoped (x=0) BaTiO3 (space group P4mm) decreases with increasing x, reaching an average cubic symmetry (space group ) at x=0.05. For x>0.05, the lattice parameter a increases almost linearly up to x=0.40, above which a secondary phase isostructural with LaYO3 precipitates. ED patterns along [110]p for 0.30≤x≤0.40 exhibit superlattice reflections at 1/2 (hkl) positions, indicating a doubling of the unit cell, which may arise from either octahedral tilting and/or chemical 1:1 B-site ordering within nanoclusters. Existence of nanodomains was revealed by dark-field transmission electron microscopy (TEM). Raman spectroscopy reveals nanoclustering to occur readily at low substitution levels and to increase progressively toward a nanoscale phase separation-like phenomenon, which precedes the precipitation of individual crystals of an LaYO3-based solid solution. Three distinctive subgrain microstructures are revealed by conventional TEM imaging. x=0 exhibits ferroelectric microdomains which completely transverse the grains, whereas for x=0.025 and 0.05, ferroelectric microdomains coexist with nanodomains. Other structural features such as core-shell-type substructures, {111} twins and dislocations are also observed in some grains. For x>0.10, only nanodomains are observed.

International audience; no abstract

International audience; The development of thin films, in the context of ongoing reduction in the size of electronic systems, poses challenging questions for the materials sciences of multifunctional nanostructures. These include the limits of size reduction, integration of heterogeneous functions, and system characterization or process control at an atomic scale. We present here different studies devoted to perovskite oxide materials (or materials with derived structure), where in specific directions of the crystal structure the atomic organization decreases down to a few nanometers, thus building nanostructures. In these materials, very original physical phenomena are observed in multilayers or superlattices, nanowires (NWs) or nanodots, mainly because strain, surfaces, and interfaces play here a predominant role and can tune the physical properties. Metal-organic chemical vapor deposition (MOCVD) routes have been used for the synthesis of oxide materials. We first introduce the basic rules governing the choice of metal-organic precursors for the MOCVD reaction. Next we discuss the principles of the pulsed injection MOCVD system. A laser-assisted MOCVD system, designed to the direct growth of 2D and 3D photonic structures, will also be described. Selected case studies will finally be presented, illustrating the powerful development of different oxide nanostructures based on dielectric, ferroelectric, or superconducting oxides, manganites, and nickelates, as well as first results related to the growth of ZnO NWs.

We present experimental results on the growth of SmNiO3 epitaxial thin films, successfully deposited by a liquid source metal-organic (MO) CVD technique. X-ray diffraction (XRD) characterizations and wavelength dispersive spectroscopy (WDS) are performed in order to optimize the composition and the purity of the films. Depositions on Si (001) and LaAlO3 (001) substrates show that the SmNiO3 phase needs a perovskite substrate with a good lattice match in order to be epitaxially stabilized. Texture analysis performed on asymmetric diffraction lines reveals the epitaxial character of the growth; in-plane and out-of plane mosaicity are determined on a 1000 A thick film.

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.

Weber, M.C. et al., Strain effects on epitaxial thin films of LaNiO3 grown on different single crystalline substrates are studied by Raman scattering and first-principles simulation. New Raman modes, not present in bulk or fully-relaxed films, appear under both compressive and tensile strains, indicating symmetry reductions. Interestingly, the Raman spectra and the underlying crystal symmetry for tensile and compressively strained films are different. Extensive mapping of LaNiO3 phase stability is addressed by simulations, showing that a variety of crystalline phases are indeed stabilized under strain which may impact the electronic orbital hierarchy. The calculated Raman frequencies reproduce the principal features of the experimental spectra, supporting the validity of the multiple strain-driven structural transitions predicted by the simulations., J.K., M.W., M.G., and J.I. acknowledge support from the National Research Fund, Luxembourg through a Pearl grant (Grant No. FNR/P12/4853155). ICMAB-CSIC authors acknowledge financial support from the Spanish Ministry of Economy and Competitiveness, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV-2015- 0496) and the MAT2014-56063-C2-1-R and MAT2013- 40581-P projects, and from Generalitat de Catalunya (2014 SGR 734). UB authors acknowledge financial support from the Spanish Ministry of Economy and Competitiveness, project MAT2013-41506-P and from Generalitat de Catalunya (2014 SGR 672).