arXiv subjects

Mael Guennou

Publications and source records attributed to Mael Guennou.

At least 19 recordsLinked to original sources

arXiv2025

An underdog story: Re-emergence of a polar instability at high pressure in KNbO3

Ferroelectricity in perovskites is known to be suppressed by a moderate hydrostatic pressure. The notion that a polar instability should reappear in a higher pressure regime is well accepted theoretically but experiments have failed so far to provide a conclusive evidence for it. Here, we investigate a classical but comparatively underlooked ferroelectric perovskite KNbO3. We use single crystal X-ray diffraction, infrared and Raman spectroscopy and second-harmonic generation to explore the phase transition sequence at high pressures up to 63 GPa. We show that the ferroelectric instability manifests itself in the emergence of an incommensurate modulation of the perovskite structure that combines cation displacements and tilts of the oxygen octahedra. Soft modes associated to the tilts and the modulation are clearly observed along with persistent order-disorder signatures. This demonstrates the presence of the high-pressure polar instability in a lead-free perovskite in spite of the centrosymmetric character of all observed high-pressure phases.

cond-mat.mtrl-sci↗

arXiv2025

Band gap tuning by structural phase transition in Sm-substituted BiFeO3 powders

The substitution of bismuth by samarium in BiFeO3 is known to induce a structural phase transition from the polar phase to a non-polar phase, with a possible antiferroelectric intermediate structure. In this paper, we investigate the impact of this phase change on the optical properties. The optical band gap was measured by diffuse reflectance as a function of temperature for several samarium concentrations across the structural phase transition. We found that the optical band gap for each of the pure phases varies linearly with temperature and that the phase transitions are revealed by smooth transitions between those linear regimes. This allows us to quantify the contribution of the structural change in the optical absorption. We find that a difference in optical band gap of about 130meV can be attributed to the phase change. We anticipate that the same change could be obtained by applying an electric field in an antiferroelectric composition.

cond-mat.mtrl-sci↗

arXiv2025

Tunable electrocaloric effect in lead scandium tantalate through calcium doping

State-of-the-art electrocaloric cooling prototypes rely on the conventional electrocaloric effect of ferroelectric lead scandium tantalate (PbSc0.5Ta0.5O3, PST), which peaks near room temperature. Here, we demonstrate that A-site calcium doping in highly ordered PST modifies its phase transitions and enables precise tuning of the electrocaloric response. The transition temperature shifts down to 258 K and up to 319 K, depending on Ca concentration. Calorimetry under electric field, electrical polarization loops, and piezoresponse force microscopy reveal the emergence of an intermediate antiferroelectric phase stabilized for Ca $\geq$ 2\%. These results are supported by first-principles calculations. We observe a conventional electrocaloric effect for Ca $\leq$ 2\% and an inverse electrocaloric effect at higher doping ($\geq$ 2\%). Under an applied field of 110 kV cm$^{-1}$, Ca-doped PST exhibits an adiabatic temperature change of 2 K over a range from 263 K to 353 K. Such Ca-doped PST compounds could be used to expand the temperature range of PST below the freezing point of water. Our results offer a pathway to cascaded electrocaloric cooling devices with extended operating spans.

cond-mat.mtrl-sci↗

arXiv2024

Raman resonances mediated by excitonic polarons in BiVO$_4$

Excitonic polarons are quasiparticles formed by a Coulomb-bound electron-hole pair with strong coupling to lattice vibrations. Despite high fundamental interest in excitonic polarons, the experimental investigation of these particles remains challenging. In this work, we exploit the resonant Raman effect to probe the excitonic polarons in bismuth vanadate. We track enhancement of Raman modes as a function of excitation energy and reveal two optical resonances: one inside the band gap at 1.94 eV and another one near the optical absorption edge at 2.45 eV. The high-energy resonance originates from free excitons, which exhibit a characteristic 40 meV anisotropy between polarizations parallel and perpendicular to the c axis. Remarkably, the low-energy resonance shows no contrast in the optical absorption spectra. We attribute this resonance to an excitonic polaron formed through strong exciton-phonon coupling, making excitonic and excitonic polaron Raman resonances similar in strength. We probe the energy level of the excitonic polaron and compare its coupling strength to the different vibrational modes. Our results establish resonant Raman spectroscopy as a unique and powerful tool for probing quasiparticles of polaronic and excitonic nature in oxide materials.

cond-mat.mtrl-sci↗

arXiv2024

Lattice-dynamics and in-plane antiferromagnetism in MnxZn1_xPS3 across the entire composition range

Alloyed MnxZn1_xPS3 samples have been grown covering the whole compositional range and studied by means of Raman spectroscopy at temperatures covering from 4K up to 850K. Our results, supported by SQUID magnetic measurements, allowed, from one hand, to complete the magnetic phase diagram of MnxZn1_xPS3 and establish x>0.3 as the composition at which the alloy retains antiferromagnetism and, from the other hand, to identify the Raman signatures indicative of a magnetic transition. The origin of these Raman signatures is discussed in terms of spin-phonon coupling resulting in the appearance of low- and high-frequency zone-folded phonon modes. For the alloy, an assignment of the 1st and 2nd order modes is provided with the aid of first-principle lattice-dynamical calculations. The compositional dependence of all phonon modes is described and the presence of zone-folded modes is shown to take place for both, the alloy and MnPS3. Finally, a comparison of the Raman spectra of ZnPS3 to other compounds of the transition-metal phosphorous trisulfide family allowed shows that low-frequency phonon peaks exhibit an abnormally large broadening. This is consistent with previous claims on the occurrence of a second-order Jahn-Teller effect that takes place for ZnPS3 and Zn-rich MnxZn1_xPS3.

cond-mat.mtrl-sci↗

arXiv2023

Anharmonicity of the antiferrodistortive soft mode in barium zirconate BaZrO$_3$

Barium zirconate (BaZrO$_3$) is one of the very few perovskites that is claimed to retain an average cubic structure down to \SI{0}{\K}, while being energetically very close to an antiferrodistortive phase obtained by condensation of a soft phonon mode at the R point of the Brillouin zone boundary. In this work, we report a combined experimental and theoretical study of the temperature dependence of this soft phonon mode. Inelastic neutron and x-ray scattering measurements on single crystals show that it softens substantially from \SI{9.4}{\meV} at room temperature to \SI{5.6}{\meV} at \SI{2}{\K}. In contrast, the acoustic mode at the same R point is nearly temperature independent. The effect of the anharmonicity on the lattice dynamics is investigated non-perturbatively using direct dynamic simulations as well as a first-principles based self-consistent phonon theory, including quantum fluctuations of the atomic motion. By adding cubic and quartic anharmonic force constants, quantitative agreement with the neutron data for the temperature dependence of the antiferrodistortive mode is obtained. The quantum fluctuations of the atomic motion are found to be important to obtain the proper temperature dependence at low temperatures. The mean squared displacements of the different atoms are determined as function of temperature and are shown to be consistent with available experimental data. Adding anharmonicity to the computed fluctuations of the Ba-O distances also improves the comparison with available EXAFS data at \SI{300}{\K}.

cond-mat.mtrl-sci↗

arXiv2023

Phonon-phonon coupling in bismuth vanadate over a large temperature range across the monoclinic phase

In this work we study phonon-phonon coupling in bismuth vanadate (BiVO4), known for its second-order transition involving a variety of coupling mechanisms. Using Raman spectroscopy as a probe, we identify two optical coupled phonon modes of the VO4 tetrahedron and study them by varying light polarization and temperature. The coupling manifests in non-Lorentzian line-shapes of Raman peaks and frequency shifts. We use theoretical framework of coupled damped harmonic oscillators to model the coupling and capture the phenomena in the temperature evolution of the coupling parameters. The coupling is negligible at temperatures below 100 K and later increases in magnitude with temperature until 400 K. The sign of the coupling parameter depends on the light polarization direction, causing either phonon attraction or repulsion. After 400 K the phonon-phonon coupling diminishes when approaching phase transition at which the phonon modes change their symmetry and the coupling is no longer allowed.

cond-mat.mtrl-sci↗

arXiv2022

Lattice dynamics and Raman spectrum of supertetragonal PbVO3

Lead vanadate PbVO3 is a polar crystal with a P4mm space group at ambient conditions. It is isostructural with the model soft-mode driven ferroelectric PbTiO3, but differs from it by the so-called 'supertetragonal' elongation of its unit cell. In this paper, we report a combined study of the lattice dynamics of PbVO3 by Raman spectroscopy at room temperature and first-principle calculations. All zone-center transverse optical (TO) phonon modes are identified by polarized, angle-dependent Raman spectroscopy and assigned as follows: E modes at 136, 269, 374 and 508 cm-1, A1 modes at 188, 429 and 874 cm-1 and B1 mode at 319 cm-1. The calculations confirm the experimental symmetry assignment and allow to obtain the longitudinal (LO) phonons wavenumbers. Besides, we analyze the mode eigenvectors in detail, in order to identify the atomic displacements associated with each mode and compare them with PbTiO3. In spite of their differences in chemistry and strain, the phonon eigenvectors are found to be remarkably comparable in both compounds. We discuss the position of the ferroelectric soft mode in PbVO3 as compared to PbTiO3. A sizeable splitting of the B1+E modes appears as a characteristic feature of supertetragonal phases. The peculiarity of the vanadyl V-O bond frequency in PbVO3 is also addressed.

cond-mat.mtrl-sci↗

arXiv2022

Stability of the tetragonal phase of BaZrO3 under high pressure

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.

cond-mat.mtrl-sci↗

arXiv2022

Birefringence induced by antiferroelectric switching in transparent polycrystalline $PbZr_{0.95}Ti_{0.05}O_{3}$ film

The most characteristic functional property of antiferroelectric materials is the possibility to induce a phase transition from a non-polar to a polar phase by an electric field. Here, we investigate the effect of this field-induced phase transition on the birefringence change of $PbZr_{0.95}Ti_{0.05}O_{3}$. We use a transparent polycrystalline $PbZr_{0.95}Ti_{0.05}O_{3}$ film grown on $PbTiO_{3}/HfO_{2}/SiO_{2}$ with interdigitated electrodes to directly investigate changes in birefringence in a simple transmission geometry. In spite of the polycrystalline nature of the film and its moderate thickness, the field-induced transition produces a sizeable effect observable under a polarized microscope. The film in its polar phase is found to behave like a homogeneous birefringent medium. The time evolution of this field-induced birefringence provides information about irreversibilities in the antiferroelectric switching process and its slow dynamics. The change in birefringence has two main contributions, one that responds briskly (~ 0.5 s), and a slower one that rises and saturates over a period of as long as 30 minutes. Possible origins for this long saturation and relaxation times are discussed.

cond-mat.mtrl-sci↗

arXiv2020

Critical field anisotropy in the antiferroelectric switching of PbZrO3 films

Antiferroelectrics have been recently sparking interest due to their potential use in energy storage and electrocaloric cooling. Their main distinctive feature is antiferroelectric switching, i.e. the possibility to induce a phase transition to a polar phase by an electric field. Here we investigate the switching behavior of the model antiferroelectric perovskite PbZrO3 using thin films processed by chemical solution deposition in different geometries and orientations. Both out-of-plane and in-plane switching configurations are investigated. The critical field is observed to be highly dependent on the direction of the electric field with respect to the film texture. We show that this behaviour is qualitatively consistent with a phase transition to a rhombohedral polar phase. We finally estimate the importance of crystallite orientation and film texturation in the variations observed in the literature.

cond-mat.mtrl-sci↗

arXiv2020

Role of the ferroelastic strain in the optical absorption of BiVO4

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 band gap is found to be very sensitive to temperature, and to the monoclinic-to-tetragonal ferroelastic transition at 523K. The anisotropy, as measured by the difference in absorption edge for 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.

cond-mat.mtrl-sci↗

arXiv2020

High-contrast imaging of 180° ferroelectric domains by optical microscopy using ferroelectric liquid crystals

Ferroelectric liquid crystals (FLCs) couple the direction of their spontaneous electric polarization to the direction of tilt of their optic axis. Consequently, reversal of the electric polarization by an electric field gives rise to an immediate and lasting optical response when an appropriately aligned FLC is observed between crossed polarizers, with one field direction yielding a dark image, and the opposite direction yielding a bright image. Here this peculiar electro-optic response is used to image, with high optical contrast, 180° ferroelectric domains in a crystalline substrate of magnesium-doped lithium niobate. The lithium niobate substrate contains a few domains with upwards electric polarization surrounded by regions with downward electric polarization. In contrast to a reference non-chiral liquid crystal that is unable to show ferroelectric behavior due to its high symmetry, the FLC, which is used as a thin film confined between the lithium niobate substrate and an inert aligning substrate, reveals ferroelectric domains as well as their boundaries, with strong black and white contrast. The results show that FLCs can be used for non-destructive read-out of domains in underlying ferroelectrics, with potential applications in e.g. photonic devices and non-volatile ferroelectric memories.

cond-mat.mtrl-sci↗

arXiv2020

Imaging of Strain Driven Magnetic Domains and Strong Spin-Phonon Coupling in Epitaxial Thin Films of SrRuO3

Epitaxial thin films of SrRuO3 with large strain disorder were grown using pulsed laser deposition method which showed two distinct transition temperatures in Magnetic measurements. For the first time, we present visual evolution of magnetic domains across the two transitions using Magnetic force microscopy on these films. The study clearly showed that the magnetic anisotropy corresponding to the two transitions is different. It is observed that the perpendicular magnetic anisotropy is dominating in films which results in domain spin orientation preferably in out of plane direction. The Raman studies showed that the lattice is highly influenced by the magnetic order. The analysis of the phonon spectra around magnetic transition reveals the existence of strong spin-phonon coupling and the calculations resulted in spin-phonon coupling strength (λ) values of λ ~ 5 cm-1 and λ ~ 8.5 cm-1, for SrRuO3 films grown on LSAT and SrTiO3 single crystal substrates, respectively.

cond-mat.mtrl-sci↗

arXiv2019

Archetypal soft-mode driven antipolar transition in francisite Cu3Bi(SeO3)2O2Cl

Model materials are precious test cases for elementary theories and provide building blocks for the understanding of more complex cases. Here, we describe the lattice dynamics of the structural phase transition in francisite Cu3Bi(SeO3)2O2Cl at 115 K and show that it provides a rare archetype of a transition driven by a soft antipolar phonon mode. In the high-symmetry phase at hightemperatures, the soft mode is found at (0,0,0.5) at the Brillouin zone boundary and is measured by inelastic X-ray scattering and thermal diffuse scattering. In the low-symmetry phase, this softmode is folded back onto the center of the Brillouin zone as a result of the doubling of the unit cell, and appears as a fully symmetric mode that can be tracked by Raman spectroscopy. On both sides of the transition, the mode energy squared follows a linear behaviour over a large temperature range. First-principles calculations reveal that, surprisingly, the flat phonon band calculated for the high-symmetry phase seems incompatible with the displacive character found experimentally. We discuss this unusual behavior in the context of an ideal Kittel model of an antiferroelectric transition.

cond-mat.mtrl-sci↗

arXiv2019

Lattice dynamics and Raman spectrum of BaZrO3 single crystals

BaZrO3 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 BaZrO3 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 BaZrO3, 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 modes 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.

cond-mat.mtrl-sci↗

arXiv2018

Raman signatures of ferroic domain walls captured by principal component analysis

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.

cond-mat.mtrl-sci↗

arXiv2018

Evolution of defect signatures at ferroelectric domain walls in Mg-doped LiNbO3

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.

cond-mat.mtrl-sci↗