Low temperature heat capacity of permanently densified SiO₂ glasses

Abstract

A study of low temperature specific heat capacity (1–30 K) has been performed on samples of vitreous SiO₂, which have been compacted under pressures up to 8 GPa 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 10 K up to about 17 K of the broad hump, the calorimetric Boson peak (BP), observed above 1 K 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 α-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.

Keywords:

• glass
• specific heat
• vibrational properties

PACS number:

• 63.50.+x
• 65.40.+g
• 78.30.Ly

Acknowledgements

The authors want to thank Dr. Marco Zanatta (Dipartimento di Fisica e Geologia, Università di Perugia, Italy) for having measured the Brillouin light scattering spectra and the density of compacted glasses.

Disclosure statement

No potential conflict of interest was reported by the authors.