Pressure dependence of the intermediate-range structure and the boson peak in oxide glasses

Despite glasses are widely used for several applications, some of their unique structural and dynamical properties are not completely understood. We present a detailed investigation of the intermediate-range structure of a series of alkaline borate glasses and of densified borate glasses carried out by performing neutron diffraction measurements. Moreover, a comparative study of Raman scattering, inelastic neutron scattering measurements and low temperature specific heat capacity has been performed on densified B2O3 glasses. Strong differences are observed in the intermediate range order as a function of density, the specific alkaline ion and of its concentration. On these results, we suggest that the first sharp diffraction peak of glasses arises from the periodicity of the boundaries of interstitial voids in the random glassy network. This interpretation provides a general explanation for all the anomalous compositional and pressure dependence of the FSDP in glasses as due to changes in the distribution of the sizes of the voids. The detailed analysis of the Boson peak in densified B2O3 glasses demonstrates that all the low-energy vibrational modes giving rise to the BP, extended and localized, are coupled and hybridized, determining an overall spectral distribution, whose spectral shape does not depend on the different packing fractions of the systems. Furthermore, a clear correlation between the boson peak frequency and the transverse sound velocity is found, suggesting a mainly transverse character of the excess vibrational modes in glasses. It is indeed believed that vibrations merging into the boson peak arise from low atomic density regions and that they are strongly influenced by constraints imposed by the densification on transverse displacements of structural units overlooking these voids. A good accordance between the correlation lengths of densified glasses (estimated from the boson peak in Raman spectra and from the position of the first sharp diffraction peak), shows that the low-frequency excitations are defined by the characteristic length of voids in the glassy structure. Lastly, a set-up for in-situ high-pressure Raman experiments was developed, allowing to analyse the low energy modes of a sample of α-quartz at increasing pressures up to 26 GPA. The pressurization on the crystalline sample by a Diamond Anvil Cell provokes its amorphyzation.