We reinvestigate sound dispersion and attenuation in a SiO2 layer as a function of temperature over the range 20–300 K by picosecond acoustics [1]. A pulse-echo scheme is used, where a broadband strain-pulse (centered at 120 GHz) is detected in an Al transducer after propagating back and forth through the SiO2 layer. The acoustic attenuation coefficient α within the SiO2 layer is evaluated by fitting the echoes to a mismatch model including an effective local law for the frequency dependence of attenuation over the band of the pulse. In this way, the T dependence of α in SiO2 layers could be extracted in this work for the first time. Results are found to follow rather well a model combining coupling to thermally activated relaxation mechanisms and interactions with thermal vibrations. This leads to a non-trivial variation of the attenuation coefficient with frequency and temperature. The number density of relaxing defects in the SiO2 layer is found to be slightly higher than that in bulk v-SiO2. In contrast, similar anharmonic contribution to acoustic absorption is observed in both systems. The velocity variations are also measured and are compared to the dynamical velocity changes deduced from the sound attenuation.

The talk focuses on the thermodynamical properties of vitreous silica submitted to high pressures in a diamond anvil cell as obtained directly from Brillouin Light Scattering experiments or indirectly from standard relations. The analysis reveals non-negligible differences between static and dynamic compressibilities which are mostly related to the existence of thermally activated relaxational processes. Estimate of the residual densifications after complete cycles of compression/decompression is discussed.

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The vibrational properties of three sodosilicate glasses have been investigated in the framework of Density Functional Theory. The Raman spectra are calculated and the results compare well with the experimental observations. The vibrational analysis confirms the presence of Si-O-Si bending as well as breathing modes of small rings at intermediate frequencies. The Qn Raman-feature at high frequency is decomposed into Q2, Q3, and Q4 vibrational species. Interestingly, the results suggests a large overlap between the three contributions as well as spectral shapes that are not necessarily unimodal nor Gaussian, as frequently assumed in the treatment of the experimental data. Raman scattering of ternary alumino-silicate glasses with alkali and alkaline-earth cations (Mg, Ca, Sr, Ba, Na,...) has also been performed. In these glasses, vibrational signatures of cations are clearly evidenced at low frequency in the depolarized (VH) spectra. Raman scattering by cations at low-frequency is confirmed by the computational data in the sodo-silicates. In the alumino-silicate glasses the responses associate to different types of motions. The analysis allows separating the contribution arising from network modifier cations to that originating from charge compensator ones.

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.