arXiv subjects

Cosme Milesi-Brault

Publications and source records attributed to Cosme Milesi-Brault.

7 recordsLinked to original sources

arXiv2026

Consistent transition model for Bi0.5Na0.5TiO3 from temperature-dependent structural and electrical properties

BNT based solid solutions are promising parent materials for lead free dielectric capacitors, thanks to their high recoverable energy densities and breakdown strengths. However, the ambient temperature symmetry and high temperature phase evolution of BNT remain unclear. Crucially, structural transformations and electrical ordering are most often considered independently, hindering a coherent understanding of the BNT phase transition. In this work, we combine X ray diffraction, transmission electron microscopy, Raman spectroscopy, impedance spectroscopy, and high field polarization cycling to establish a unified picture of the structural and dielectric response of BNT. Based on these results, we propose a consistent transition model for BNT that reconciles previously conflicting interpretations. This integrated structure property study provides a rationale for developing high performance, lead free energy storage materials.

cond-mat.mtrl-sci↗

arXiv2025

Phonon studies of the phase transition sequence in antiferroelectric single crystal of Pb(Hf0.83Sn0.17)O3

The sequence of phase transitions in PbHf0.83Sn0.17O3 has been studied by THz, far infrared and Raman spectroscopies, revealing the complementary behaviour of both, polar and non-polar phonons and their impact on the transition lattice dynamics. Pb atom is sensitive to all phase transitions, changing its dynamics with temperature. As temperature decreases, the crystal undergoes a sequence of three phase transitions. The first one to an intermediate (IM) phase, in which polar fluctuations are detected by THz and IR spectroscopy at frequencies below 100 cm-1 contributing to the maximum of permittivity and revealing important softening. Additional antipolar Pb fluctuations and softening were detected by Raman spectroscopy. At lower temperature, another transition to an antiferroelectric (AFE2) phase is revealed by nonpolar soft modes (antipolar and antiferrodistortive ones), and by an important drop of the dielectric strength of polar phonons. The final transition to the antiferroelectric phase (AFE1) is revealed by the appearance of new modes and a sudden change in the frequency of the soft modes when the antipolar shifts become larger. Using symmetry analysis and optical observation to study how the domain pattern changes with temperature, we identified a path for the cubic-AFE2 transition throughout the IM phase, of plausible tetragonal symmetry, driven by an instability from the center of the Brillouin zone. This mechanism coexists with antiferrodistortive instabilities that eventually drive the material into the AFE2 phase. The final phase transition to the AFE1 phase naturally follows from a mode outside the center of the Brillouin zone and a further doubling of the unit cell.

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↗

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↗

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↗

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↗