The equation of state of polyamorphic germania glass: A two-domain description of the viscoelastic response

We have measured the quasistatic room temperature equation of state of GeO₂ glass under hydrostatic conditions to 7.1 GPa. From ambient pressure to 4 GPa the compression displays completely reversible elastic behavior. Above 4 GPa the glass becomes anelastic and exhibits a dramatic increase in the static compressibility. This change in elastic response is concomitant with the onset of the previously reported pressure-induced germanium coordination change. The equation of state data can be quantitatively described by a two-domain model composed of four- and six-coordinated germanium clusters. The model accurately reproduces the previously measured change in the average Ge-O bond length of germania with pressure and rationalizes the different pressure dependent compressional behavior observed in quasistatic and ultrasonic measurements. We further conjecture that the vitreous polyamorphism exhibited by germania glass at high pressures, and the pressure-induced crystal-to-amorphous transition of quartz-isotypic GeO₂, both result from similar underlying coordination instabilities in the germania tetrahedral framework.