High-pressure polarized Raman spectra of vitreous silica are measured up to 8 GPa in a diamond-anvil cell atroom 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.

Single crystals of α-quartz type Si1−xGexO2 with x < 0.2 were grown in 0.05 M NaOH solution. Infrared measurements confirmed that crystals grown at high pressure and high temperature have a low –OH group defect content. After a few millimeters of crystal growth under these conditions, α3500 reaches 0.1 cm−1 and no free –OH are present. The d11 value measured on an X-cut from the crystal with x = 0.0375 is 3.08 pC N−1 (±0.15), in good agreement with the value of 2.97 pC N−1 obtained using density functional theory based calculations. Piezoelectric measurements were performed on the Si1−xGexO2 crystal with x = 0.0375, both at room temperature and after annealing at various temperatures. The piezoelectric signal of the crystal with x = 0.0375 and pure SiO2 disappears after annealing at 635 °C and 545 °C, respectively. Nonlinear optical (NLO) properties of Si1−xGexO2 crystals were measured by Maker's fringe technique on Z-cut χ11(2) and their corresponding values for x = 0.023 and 0.028 are 1.3(2) pm V−1 and 1.6(2) pm V−1, respectively. These values are in good agreement with density functional theory based calculations. The light induced damage threshold values measured on Si1−xGexO2 crystals with x = 0.023 and 0.028 are very similar to that of α-quartz.

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.

The results obtained by high pressure neutron powder diffraction and single-crystal x-ray diffraction for the P4mm–Pm $\bar{3}$ m phase transition in the prototype ferroelectric perovskite lead titanate are shown. Neutron diffraction is found to be strongly sensitive to the dipolar moment in the PbTiO3 unit cell due to the gradual reduction of the displacement of the Ti and O atoms from centrosymmetric positions in the cubic perovskite structure which exhibits anti-phase scattering of Pb, Ti and O atoms. From applying both techniques, the anomalously high Debye–Waller factor for the lead atoms confirms the disordered character of the cubic phase. High pressure single crystal x-ray diffraction also perfectly describes the ferroelectric–paraelectric transition and will be the technique of choice to solve higher pressure structures for PbTiO3.