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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From the new infrared (IR) reflectivity and time-domain terahertz spectra combined with available highfrequencydielectric data above the megahertz range in a broad temperature range of 10 to 900 K, a fullpicture of the soft- and central-mode behavior in the classical relaxor ferroelectric Pb(Mg1/3Nb2/3)O3 (PMN) issuggested. A detailed comparison is made with the recent hyper-Raman spectroscopy data [Hehlen et al., Phys.Rev. Lett. 117, 155501 (2016)] and also with other available experiments based on inelastic light and neutronscattering. It is revealed that each type of experiment provides slightly different data. The closest agreementis with the hyper-Raman data: both techniques yield the same number of soft-mode components and the samehigh-temperature softening towards the temperature T∗ ≈ 400 K. In addition to evaluation of the IR-terahertz datausing fitting with the standard factorized form of the dielectric function, we performed a successful fitting of thesame data using the effective medium approach (EMA), originally based on the assumption that the mesoscopicstructure of PMNconsists of randomly oriented uniaxially anisotropic polar nanodomains (PNDs) with somewhatharder transverse optical polar modes in the direction along the local PND dipole [Hlinka et al., Phys. Rev. Lett.96, 027601 (2006)]. Evaluation using Bruggeman EMA modeling has been successfully applied in the entireinvestigated temperature range. These results suggest that the response perpendicular to the local dipole moment,at high temperatures induced by random fields rather than PNDs, undergoes a classical softening from hightemperatures with permittivity obeying the Curie-Weiss law, ε⊥ = C/(T − TC), where C = 1.7 × 105K andTC = 380 K, whereas the response parallel to it shows no softening. Below the Burns temperature, ∼620 K, agigahertz relaxation ascribed to flipping of the PNDs emerges from the soft-mode response, slows down, andbroadens, remaining quite strong towards the cryogenic temperatures, where it can be assigned to fluctuations ofthe PND boundaries.

Hyper-Raman scattering (HRS) is a non-linear spectroscopy sensitive to all polar excitations, in particular the soft modes inactive in Raman. HRS has been applied to nano-ceramics of strontium titanate (SrTiO3) of controlled grain size. Contrary to infrared absorption which sees an average medium, the vibrational responses of the core and the shell(s) are split in HRS, allowing thereby probing the structural and dielectric properties of the two subsystems. The resulting structural model successfully reproduces the effective dielectric permittivity measured by dielectric experiments [1]. These results confirm the strong, but still under-exploited, potentialities of HRS for the investigation of polar materials [2].We also demonstrate that a new ground state can be obtained by tailoring the core-shell structure of the particles. High energy X-rays at synchrotron combined to neutron diffraction and HRS revealed a lowering of the ferroelastic ground state towards a new antiferrodistortive phase, accompanied with strong shift of the critical temperature [3,4]. This new phase is discussed within the Landau theory, and the crucial competition between particle shape anisotropy, surface tension, and shear strain is analyzed as well. This shows that controlling the core-shell structure provides an easy way to stabilize new phases that cannot exist in bulk material, just like film deposition on a substrate.

Density functional theory is used to calculate the vibrational properties of pure silica and sodo-silicateglasses with 20, 25, and 33 mol% of Na2O. The infrared and Raman spectra are calculated and the full responses aredecomposed into principal structural components (PSC). Those are for example the SiO4n--tetrahedra with n nonbrigdingoxygens defining the Qn-species at the origin of the structured feature at high frequency, and the Si-O-Sibridges leading the broad Raman R-band at intermediate frequencies. Our results confirm that Si-O-Si bending inbridges with large angle vibrate preferentially at lower frequencies than those with low angle. In addition, the spectralresponse of the Q2-species is bimodal and overlaps with that of the Q3, while the Q4 response covers almost all ofthe spectral range of the Qn-band. The ab-initio individual spectral responses of the PSC are used to reconstruct theexperimental Raman responses. Contrary to the commonly used multi-Gaussian decomposition, this approachprovides unambiguous band-assignments and hence a more accurate way to probe the structural and chemicalproperties of glasses from their spectroscopic signature.