Hybrid organic-inorganic perovskites are intensively studied as promising materials for various applications, taking advantage of their low cost and easy processing. Although much recent attention has been devoted towards unraveling their microscopic optoelectronic properties, the low frequency dynamics (e.g. their softness) is currently still lacking a comprehensive understanding as compared to that already reached for classic semiconductors. Here we investigate the low frequency dynamics in four different hybrid perovskite single crystals: MAPbBr3, FAPbBr3, MAPbI3 and α-FAPbI3. Using coherent inelastic neutron scattering spectroscopy and Brillouin light scattering, we investigate the low frequency acoustic phonons [1,2]. A very small shear C44 elastic constant is found for all the compounds as well as sizeable elastic anisotropy. The extremely low bulk modulus and negative C12 in α-FAPbI3 substantiates its very unstable nature and in FAPbBr3, a tendency towards an incipient ferroelastic transition, is interpreted as further evidence of the influence of plasticity in hybrid perovskites. We observe a systematic lower sound group velocity in the technologically important iodide-based compounds compared to the bromide-based ones [2]. The findings suggest that low thermal conductivity and hot phonon bottleneck phenomena are expected to be enhanced by low elastic stiffness, particularly in the case of the ultrasoft α-FAPbI3 (Fig. 1a).The low-frequency dynamics is further investigated by Raman spectroscopy. The scattering from the low-frequency optical phonons has been collected across the cubic to tetragonal transition. With the resolution and contrast available by our experiment (~4-5 cm-1 at the bottom of the elastic line) we do not observe the divergence of a central peak. However, the lowest frequency vibration, almost temperature independent in the cubic phase, hardens on cooling in the tetragonal phase. These conclusions apply in three of the four samples (analysis in α-FAPI still in progress) suggesting a common framework for the analysis of the low-frequency vibrational dynamics of this family compounds (Fig. 1b). In CH3NH3PbI3 the Raman spectroscopy across the full set of phase transitions down to 20 K will also be discussed.

Raman scattering of ternary alumino-silicate glasses with alkali and alkaline-earth cations(M=Mg, Ca, Sr, Ba, Na,...) has been performed. The concentrations range from the peral-caline (MO/Al2O3> 1) to the peraluminate (MO/Al2O3< 1) domain keeping constant thesilica content. The vibrational signature of the cations is clearly evidenced at low frequencyin the depolarized (VH) spectra. Between one and three bands are identifed. They associate to different types of motions depending on the charge carried by the cation (alkali oralkaline-earth atoms). In addition, glasses with magnesium exhibit a very peculiar behavior.Very interestingly, some bands disappear for the tectosilicate glasses (MO/Al2O3=1) andin the peraluminate domain revealing a clear vibrational contrast between network modifer cations and charge compensator cations. A preliminary experiment in a quaternaryalumino-silicate system combining two types of alkali atoms highlights the possibility by sim-ple Raman spectroscopy to define the preferential role of each cation, i.e. network modieror charge compensator, and to make a semi-quantitative treatment as well.

The paper will discuss the possibility for light spectroscopies, e.g. Brillouin Light Scat- tering or Raman Scattering, to locally monitor the densification of amorphous silica. It is indeed well-known that silica significantly densifies under high stresses, either elastically as for example during a high-pressure experiment in a diamond-anvil cell below about 10 GPa or plastically above that limit. Further, this elastic limit strongly depends on the temperature. Irradiation-induced density changes are also observed using ultra-short laser pulses, neutrons or other particles. All these different routes lead to substantial structural modifications which in turn alter the response of silica to light spectroscopy in a complex manner.

High-pressure Brillouin [1] and polarized Raman spectra [2] of vitreous silica are measured up to 8 GPa in diamond-anvil cell at room temperature. On one hand, Brillouin frequencies show 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. On the other hand, the combined use of either a nonpenetrating pressurizing medium-argon-or a penetrating one-helium and Neon, allows one to separate density from stress effects on the Raman frequencies.[1]C. Weigel, A. Polian, M. Kint, B. Ruffl ́e, M. Foret, and R. Vacher, Phys. Rev. Lett. 109, 245504 , 2012[2]C. Weigel, M. Foret, B. Hehlen, M. Kint, S. Cl ́ement, A. Polian, R. Vacher, and B. Ruffl ́e, Phys. Rev. B 93, 224303 , 2016

Silica exists under a number of different forms, either crystalline or vitreous. The density of silica glass is about 5% lower than that of cristobalite and 20% lower than that of quartz, two crystals of the same chemical composition. Further, vitreous silica can be permanently densified under high pressure and high temperature by more than 20%, keeping an amorphous structure. What is the origin of these differences at the microscopic scale ? We will first review our knowledge of the structure of silica glass : the basic element of the structure is a SiO4 tetrahedron as in crystals, but the angles between tetrahedra are irregular. Then, results of experiments for silica glass under compression will be presented. Under pressures in the GPa range, the results depend drastically on the fluid used as a pressure-transmitting medium for the compression : while the volume of the silica sample changes dramatically in argon, only a very small decrease in size is observed under helium. This shows that helium penetrates inside silica, the open and flexible structure of the glass allowing gas atoms to distend the network, the same way a porous material deforms upon fluid adsorption. Furthermore, a fine quantitative analysis of the behavior of the Raman modes in function of pressure shows that changes in force constants between atoms and bond angles between tetrahedra in the network occur simultaneously.

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