Low temperature heat capacity of permanently densified SiO2 glasses

A study of low temperature specific heat capacity (1-30K) has been performed on samples of vitreous SiO2, which have been compacted under pressures up to 8GPa to explore different glassy phases having growing density. Increasing densification by more than 21% leads to a progressive reduction of the specific heat capacity C-p and to a shift from 10K up to about 17K of the broad hump, the calorimetric Boson peak (BP), observed above 1K in a C-p(T)/T-3 vs. T plot. The revealed changes are not accounted for by the modifications of the elastic continuum, implying a nature of additional vibrations at variance with the extended sound waves. Increasing atomic packing of the glassy network leads to a progressively decreasing excess heat capacity over that of alpha-quartz, a crystalline polymorph of SiO2. By using the low-frequency Raman intensity measured in these glasses to determine the temperature dependence of the low temperature heat capacity, it has been evaluated the density of low-frequency vibrational states. The observations are compared with some theoretical pictures explaining the nature of the BP, disclosing qualitative agreement with the predictions of the Soft Potential Model and the results of a simulation study concerning the vibrations of jammed particles. This finding leads to evaluate a nanometer length scale which suggests the existence of poorly packed domains formed from several n-membered rings involving SiO4 tetrahedra. These soft regions are believed to be the main source of low-frequency vibrations giving rise to the BP.