We present accurate measurement of the VV- and VH-polarized Raman spectra of vitreous SiO2 pressurized in the range 0 – 8 GPa in argon or helium atmosphere by means of diamond anvil cell. A fine quantitative analysis of the behavior of the Raman modes in function of pressure is achieved with the help of an old central-force model by Sen and Thorpe describing the dynamics of covalently bonded networks. The model predicts the effect of changes in force constants and bond angles on the vibrational frequencies of the coupled SiO4 tetrahedral network. We show that the pressure shift of the Raman bands cannot be understood without encompassing the variations of both parameters. Investigating two different pressurizing fluids including He which is adsorbed into the silica network, allows revealing the role of each parameter in the compression mechanism of the glass.

High-pressure polarized Raman spectra of vitreous silica are measured up to 8 GPa in a diamond-anvil cell at room temperature. The combined use of either a nonpenetrating pressurizing medium—argon—or a penetrating one—helium, allows one to separate density from stress effects on the Raman frequencies. In the framework of a simple central force model, the results emphasize the distinct role played by the shrinkage of the intertetrahedral angle Si-O-Si and the force-constant stiffening during the compression. The polarization analysis further reveals the existence of an additional isotropic component in the high-frequency wing of the boson peak. The pressure dependence of the genuine boson peak frequency is found to be much weaker than previously reported and even goes through a minimum around 2 GPa in remarkable coincidence with the anomalous compressibility maximum of silica.

Despite 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.

In this paper, we examine recent theoretical investigations on 3D hybrid perovskites (HOP) that combine concepts developed for classical bulk solid-state physics and empirical simulations of their optoelectronic properties. In fact, the complexity of HOP calls for a coherent global view that combines usually disconnected concepts, related to classical semiconductors, perovskites and plastic crystals1. For the pseudocubic high temperature reference perovskite structure that plays a central role for 3D HOP, we introduce a new tight-binding Hamiltonian, which specifically includes spin-orbit coupling. The resultant electronic band structure is compared to that obtained using state of the art density functional theory (DFT). Possible plastic crystal and orientational glass behaviors of HOP will be discussed, building on Car-Parrinello molecular dynamics simulations and symmetry analyses of the cation stochastic reorientations. As available experimental or simulated data on low energy structural excitations are limited, large single crystals of MAPbBr3 have been grown and used to perform neutron and Brillouin scattering and ultrasonic characterization of low frequency acoustic phonons, as well as relaxation modes in different energy ranges. Neutron scattering experiments further reveal that the MAPbBr3 crystal presents a coexistence of crystalline domains below the cubic to tetragonal phase transition at Tc=225K. This observation is consistent with the improper ferroelastic character of the phase transition as predicted from the symmetry analysis of phonon and molecular relaxational modes. [1]Even, J., Carignano, M. & Katan, C. “Molecular disorder and translation/rotation coupling in the plastic crystal phase of hybrid perovskites.” Nanoscale DOI: 10.1039/C5NR06386H, (2016)

La diffusion hyper-Raman (DHR) est une spectroscopie optique des vibrations qui présente des règles de sélection différentes du celles du Raman et de l'infrarouge. Après avoir décrit les principes de la DHR, nous ferons un état de l'art de la dynamique vibrationnelle (IR, Raman, neutrons) du système relaxeur modèle Pb1/3Mg2/3NbO3 (PMN). Nous insisterons sur le comportement des modes mous polaires dont l'observation est uniquement possible par diffusion hyper-Raman. Les résultats ont été capturés dans un modèle qui offre une vision cohérente des propriétés dynamiques de toute une famille de matériaux. Ils relient par exemples les vibrations à la structure polaire locale, et donnent une origine microscopique aux propriétés diélectriques géantes de ces systèmes.

Une introduction à la théorie de la réponse linéaire nous permettra dans un premier temps de définir les notions de susceptibilité, de spectre des fluctuations, de fonction d’auto-corrélation,… et de relier ces grandeurs à travers le théorème de fluctuation-dissipation. Nous déterminerons ensuite l’origine de la diffusion de la lumière et calculerons l’intensité Raman et Brillouin diffusée dans le cadre d’une théorie macroscopique (classique). Ces calculs feront apparaitre des règles de sélection qui seront mises en pratique à travers quelques exemples simples pris parmi les solides cristallins et les liquides.