arXiv subjects

B. Baptiste

Publications and source records attributed to B. Baptiste.

6 recordsLinked to original sources

arXiv2025

Isostructural phase transition and equation of state of type-I and type-VIII metallic sodium borosilicide clathrates

Electronic properties of silicon-based clathrates can be tuned by boron incorporation into the silicon cage network. Sodium borosilicides clathrate outstands with uncommon stoichiometry and expected metallic properties, in contrast to other alkali metal semiconductive Zintl borosilicides. In this study, we report an experimental investigation of the high-pressure behavior of type-I and type-VIII sodium borosilicide clathrates. An isostructural phase transition, marked by an abrupt volume collapse at 13 GPa, is observed exclusively in type-I sodium borosilicide clathrates. This transition is attributed to the pressure-induced diffusion of silicon atoms from the Si(6c) site. This mechanism provides the first experimental validation of a transition predicted theoretically for this class of materials. Isostructural phase transitions were only observed in type-I borosilicide. In contrast, the type-VIII borosilicide phase exhibits conventional elastic compression. The metallic character was established using reflectance spectroscopy over a wide energy range, in good agreement with crystallographic data on the boron content.

cond-mat.mtrl-sci↗

arXiv2023

High pressure-temperature phase diagram of ammonia hemihydrate

We report a comprehensive experimental investigation of the phase diagram of ammonia hemihydrate (AHH) in the range of 2-30 GPa and 300-700 K, based on Raman spectroscopy and x-ray diffraction experiments and visual observations. Four solid phases, denoted AHH-II, DIMA, pbcc and qbcc, are present in this domain, one of which, AHH-qbcc was discovered in this work. We show that, unlike previously thought, the body-centered cubic (bcc) phase obtained on heating AHH-II below 10 GPa, denoted here as AHH-pbcc, is distinct from the DIMA phase, although both present the same bcc structure and O/N positional disorder. Our results actually indicates that AHH-pbcc is a plastic form of DIMA, characterized by free molecular rotations. AHH-qbcc is observed in the intermediate P-T range between AHH-II and DIMA. It presents a complex x-ray pattern reminiscent of the "quasi-bcc" structures that have been theoretically predicted, although none of these structures is consistent with our data. The transition lines between all solid phases as well as the melting curve have been mapped in detail, showing that: (1) the new qbcc phase is the stable one in the intermediate P-T range 10-19 GPa, 300-450 K, although the II-qbcc transition is kinetically hindered for T < 450 K, and II directly transits to DIMA in a gradual fashion from 25 to 35 GPa at 300 K. (2) The stability domain of qbcc shrinks above 450 K and eventually terminates at a pbcc-qbcc-DIMA triple point at 21.5 GPa-630 K. (3) A direct and reversible transition occurs between AHH-pbcc and DIMA above 630 K. (4) The pbcc solid stability domain extends up to the melting line above 3 GPa, and a II-pbcc-liquid triple point is identified at 3 GPa-320 K.

cond-mat.mtrl-sci↗

arXiv2021

Pressure-induced structural phase transition and suppression of Jahn-Teller distortion in the quadruple perovskite structure

By means of in situ synchrotron X-ray diffraction and Raman spectroscopy under hydrostatic pressure, we investigate the structural stability of the quadruple perovskite LaMn7O12. At 34 GPa, the data unveil a first-order structural phase transition from monoclinic I2/m to cubic Im-3 symmetry characterized by a pronounced contraction of the unit cell and by a significant modifications in the Raman phonon modes. The phase transition is also marked by the suppression of Jahn-Teller distortion which is present in the ambient monoclinic phase. In addition, above 20 GPa pressure, a sudden and simultaneous broadening is observed in several Raman modes which suggests the onset of a sizable electron-phonon interaction and incipient charge mobility. Considering that LaMn7O12 is paramagnetic insulator at ambient, and Jahn-Teller distortion is frozen in the high-pressure Im-3 phase, we argue that this phase could be a potential candidate to host a purely electronic insulator-metal transition with no participation of the lattice

cond-mat.mtrl-sci↗

arXiv2021

Orbital dichotomy of Fermi liquid properties in Sr$_2$RuO$_4$ revealed by Raman spectroscopy

We report a polarization-resolved Raman spectroscopy study of the orbital dependence of the quasiparticles properties in the prototypical multi-band Fermi liquid Sr\textsubscript{2}RuO\textsubscript{4}. We show that the quasiparticle scattering rate displays $ω^{2}$ dependence as expected for a Fermi liquid. Besides, we observe a clear polarization-dependence in the energy and temperature dependence of the quasiparticle scattering rate and mass, with the $d_{xz/yz}$ orbital derived quasiparticles showing significantly more robust Fermi liquid properties than the $d_{xy}$ orbital derived ones. The observed orbital dichotomy of the quasiparticles is consistent with the picture of Sr\textsubscript{2}RuO\textsubscript{4} as a Hund's metal. Our study establishes Raman scattering as a powerful probe of Fermi liquid properties in correlated metals.

cond-mat.str-el↗

arXiv2017

The crystal structure of Rb2Ti2O5

Recent results have demonstrated an exceptionally high dielectric constant in the range 200 K-330 K in a crystalline tianium oxide : Rb2Ti2O5. In this article, the possibility of a structural transition giving rise to ferroelectricity is carefully inspected. In particular X-Ray diffraction, high resolution transmission electron microscopy and Raman spectroscopy are performed. The crystal structure is shown to remain invariant and centrosymmetric at all temperatures between 90 K and 450 K. The stability of the C2/m structure is confirmed by DFT calculations. These important findings allow to discard the existence of a conventional ferroelectric phase transition as a possible mechanism for the unusual dielectric constant observed in this material.

cond-mat.mtrl-sci↗

arXiv2017

Resonant inelastic X-ray scattering study of spin-wave excitations in the cuprate parent compound Ca$_{2}$CuO$_2$Cl$_2$

By means of resonant inelastic x-ray scattering at the Cu L$_3$ edge, we measured the spin wave dispersion along $\langle$100$\rangle$ and $\langle$110$\rangle$ in the undoped cuprate Ca$_2$CuO$_2$Cl$_2$. The data yields a reliable estimate of the superexchange parameter $J$ = 135 $\pm$ 4 meV using a classical spin-1/2 2D Heisenberg model with nearest-neighbor interactions and including quantum fluctuations. Including further exchange interactions increases the estimate to $J$ = 141 meV. The 40 meV dispersion between the magnetic Brillouin zone boundary points (1/2,\,0) and (1/4,\,1/4) indicates that next-nearest neighbor interactions in this compound are intermediate between the values found in La$_{2}$CuO$_4$ and Sr$_2$CuO$_2$Cl$_2$. Owing to the low-$Z$ elements composing Ca$_2$CuO$_2$Cl$_2$, the present results may enable a reliable comparison with the predictions of quantum many-body calculations, which would improve our understanding of the role of magnetic excitations and of electronic correlations in cuprates.

cond-mat.str-el↗