Vitreous silica distends in helium gas: acoustic vs. static compressibilities

Sound velocities of vitreous silica are measured under He compression in the pressure range 0-6 GPa by Brillouin light scattering. It is found that the well-known anomalous maximum in the pressure dependence of the compressibility is suppressed by He incorporation into the silica network. This shows that the elastic anomaly relates to the collapse of the largest interstitial voids in the structure. The huge difference between the static and the acoustic compressibilities indicates that the amount of incorporated helium still increases at 6 GPa.Comment: 5 pages, 4 figure

Pressure-induced densification of vitreous silica: insight from elastic properties

\textit{In situ} high-pressure Brillouin light scattering experiments along loading-unloading paths are used to investigate the compressibility of vitreous silica. An accurate equation of state is obtained below \SI{9}{GPa} using sound velocities corrected for dispersion. Conversely, huge inelastic effects are observed in the range \SIrange{10}{60}{GPa}, unveiling the reversible transformation from the fourfold-coordinated structure to the sixfold one. We find that the associated density changes fully correlate with the average Si coordination number. Decompression curves from above \SI{20}{GPa} reveal abrupt backward coordination changes around \SIrange{10}{15}{GPa} and significant hysteresis. Further, contrary to common wisdom, the residual densification of recovered silica samples can be figured out from the pressure cycles.Comment: 5 pages, 4 figures, revised versio

Temperature dependence of single-crystal elastic constants of flux-grown alpha-GaPO4

International audienc

Unusual anharmonicity and hierarchy of relaxational dynamics in 3D hybrid halide perovskites

3D halide perovskites used in light emitters and PV systems exhibit an unusualanharmonicity, which is not accounted for in nowadays descriptions of phonons andelectron-phonon coupling. Therefore, the low frequency lattice vibrations andrelaxations were investigated in single crystals of the four 3D hybrid organoleadperovskites, MAPbBr3, FAPbBr3, MAPbI3, and FAPbI3, at the Brillouin zone centerusing Raman and Brillouin scattering and at the zone boundary using inelasticneutron scattering. The temperature dependence of the PbX6 lattice modes in thefour compounds can be renormalized into universal curves and no soft vibration isobserved excluding a displacive-like transitional dynamics related to structural phasetransitions. The reorientational (pseudospin) motions of the molecular cations exhibita seemingly order-disorder character recalling that of plastic crystals, but attributedto a secondary order-parameter. At ultra-low frequency, a quasi-elastic componentevidenced by Brillouin scattering and associated to the unresolved central peakobserved in neutron scattering, is attributed to center of mass anharmonic motionsand rattling of the molecular cations in the perovskite cavities. 3D halide perovskitesare therefore in a very unusual situation where anharmonicity prevails. It is leading topartial, extensive and ultraslow critical and stochatic dynamics connected or not tostructural instabilities. The extensive disorder of the lattice at normal operationtemperatures for devices is similar to premelting where only long-range acousticnormal modes survive.[1] A.Létoublon, S. Paofai, B. Ruffle, P. Bourges, B. Hehlen, T. Michel, C.Ecolivet, O.Durand, S. Cordier, C. Katan, J. Even. Elastic Constants, Optical Phonons andMolecular Relaxations in the High Temperature Plastic Phase of the CH3NH3PbBr3Hybrid Perovskite. J. Phys. Chem.Lett. 7, 3776 (2016)[2]C. Katan, A. D. Mohite, J. Even, Entropy in halide perovskites, Nature Materials,17, 377-379 (2018)[3]A. Ferreira, A. Létoublon, S. Paofai, S. Raymond, C. Ecolivet, B. Ruffle, S.Cordier, C. Katan, M. I. Saidaminov, A. A. Zhumekenov, O. M. Bakr, J. Even, P.Bourges, Elastic softness of hybrid lead halide perovskites, Phys. Rev. Lett., 121,085502 (2018)[4] A. Ferreira, S. Paofai, A. Létoublon, J. Ollivier, S. Raymond, B. Hehlen, B. Rufflé,S. Cordier, C. Katan, J. Even & P. Bourges, Direct evidence of weakly dispersed andstrongly anharmonic optical phonons in hybrid perovskites, Comm. Phys. 3, 48(2020).[5] B. Hehlen, P. Bourges, B. Rufflé, S. Clément, R. Vialla, A. C. Ferreira, C.Ecolivet, S. Paofai, S. Cordier, C. Katan, A. Létoublon, J. Even, Pseudospin-phononpretransitional dynamics in lead halide hybrid perovskites, Phys. Rev. B, 105,024306 (2022

Unusual anharmonicity and hierarchy of relaxational dynamics in 3D hybrid halide perovskites

International audience3D halide perovskites used in light emitters and PV systems exhibit an unusualanharmonicity, which is not accounted for in nowadays descriptions of phonons andelectron-phonon coupling. Therefore, the low frequency lattice vibrations andrelaxations were investigated in single crystals of the four 3D hybrid organoleadperovskites, MAPbBr3, FAPbBr3, MAPbI3, and FAPbI3, at the Brillouin zone centerusing Raman and Brillouin scattering and at the zone boundary using inelasticneutron scattering. The temperature dependence of the PbX6 lattice modes in thefour compounds can be renormalized into universal curves and no soft vibration isobserved excluding a displacive-like transitional dynamics related to structural phasetransitions. The reorientational (pseudospin) motions of the molecular cations exhibita seemingly order-disorder character recalling that of plastic crystals, but attributedto a secondary order-parameter. At ultra-low frequency, a quasi-elastic componentevidenced by Brillouin scattering and associated to the unresolved central peakobserved in neutron scattering, is attributed to center of mass anharmonic motionsand rattling of the molecular cations in the perovskite cavities. 3D halide perovskitesare therefore in a very unusual situation where anharmonicity prevails. It is leading topartial, extensive and ultraslow critical and stochatic dynamics connected or not tostructural instabilities. The extensive disorder of the lattice at normal operationtemperatures for devices is similar to premelting where only long-range acousticnormal modes survive.[1] A.Létoublon, S. Paofai, B. Ruffle, P. Bourges, B. Hehlen, T. Michel, C.Ecolivet, O.Durand, S. Cordier, C. Katan, J. Even. Elastic Constants, Optical Phonons andMolecular Relaxations in the High Temperature Plastic Phase of the CH3NH3PbBr3Hybrid Perovskite. J. Phys. Chem.Lett. 7, 3776 (2016)[2]C. Katan, A. D. Mohite, J. Even, Entropy in halide perovskites, Nature Materials,17, 377-379 (2018)[3]A. Ferreira, A. Létoublon, S. Paofai, S. Raymond, C. Ecolivet, B. Ruffle, S.Cordier, C. Katan, M. I. Saidaminov, A. A. Zhumekenov, O. M. Bakr, J. Even, P.Bourges, Elastic softness of hybrid lead halide perovskites, Phys. Rev. Lett., 121,085502 (2018)[4] A. Ferreira, S. Paofai, A. Létoublon, J. Ollivier, S. Raymond, B. Hehlen, B. Rufflé,S. Cordier, C. Katan, J. Even & P. Bourges, Direct evidence of weakly dispersed andstrongly anharmonic optical phonons in hybrid perovskites, Comm. Phys. 3, 48(2020).[5] B. Hehlen, P. Bourges, B. Rufflé, S. Clément, R. Vialla, A. C. Ferreira, C.Ecolivet, S. Paofai, S. Cordier, C. Katan, A. Létoublon, J. Even, Pseudospin-phononpretransitional dynamics in lead halide hybrid perovskites, Phys. Rev. B, 105,024306 (2022

Unusual anharmonicity and pseudospin-phonon dynamics in 3D halide perovskites

International audience3D halide perovskites used in light emitters and PV systems exhibit an unusual anharmonicity, which is not accounted for in nowadays descriptions of phonons and electron-phonon coupling. Therefore, the low frequency lattice vibrations and relaxations were investigated in single crystals of the four 3D hybrid organolead perovskites, MAPbBr3, FAPbBr3, MAPbI3, and FAPbI3, at the Brillouin zone center using Raman and Brillouin scattering and at the zone boundary using inelastic neutron scattering. The temperature dependence of the PbX6 lattice modes in the four compounds can berenormalized into universal curves and no soft vibration is observed excluding a displacive-like transitional dynamics related to structural phase transitions. The reorientational (pseudospin) motions of the molecular cations exhibit a seemingly order-disorder character recalling that of plastic crystals, but attributed to a secondary order-parameter. At ultra-low frequency, a quasi-elastic component evidenced by Brillouin scattering and associated to the unresolved central peak observed in neutron scattering, is attributed to center of mass anharmonic motions and rattling of the molecular cations in the perovskite cavities. 3D halide perovskites are therefore in a very unusualsituation where anharmonicity prevails. It is leading to partial, extensive and ultraslow critical and stochatic dynamics connected to structural instabilities. The extensive disorder of the lattice at normal operation temperatures for devices is similar to premelting where only long-range acoustic normal modes survive

Lattice dynamics in 3D lead halide hybrid perovskites

International audienceLattice dynamics in 3D lead halide hybrid perovskites is analyzed together with the possible consequences for device application

Low energy phonon spectroscopy and translation-rotation coupling in hybrid perovskites crystals

National audienceDespite the wealth of research conducted the last three years on hybrid organic perovskites (HOP), several questions remain open including: to what extend the organic moiety changes the properties of the material as compared to all-inorganic (AIP) related perovskite structures. To ultimately reach an answer to this question, we have recently introduced two approaches that were designed to take the stochastic molecular degrees of freedom into account, and suggested that the high temperature cubic phase of HOP and AIP is an appropriate reference phase to rationalize HOP's properties. In this paper, we recall the main concepts and show recent molecular dynamic results (figure 1) to discuss more specifically the various possible couplings between charge carriers and low energy excitations such as acoustic and optical phonons. As available experimental or simulated data on low energy excitations are limited, we also present preliminary neutron and Brillouin scattering, as well as ultrasonic measurements obtained on freshly prepared single crystals of CH 3 NH 3 PbBr 3