Mechanisms of anomalous compressibility of vitreous silica

Abstract

The anomalous compressibility of vitreous silica has been known for nearly a century, but the mechanisms responsible for it remain poorly understood. Using GHz-ultrasonic interferometry, we measured longitudinal and transverse acoustic wave travel times at pressures up to 5 GPa in vitreous silica with fictive temperatures $({T}_{f})$ ranging between 985 \ifmmode^\circ\else\textdegree\fi{}C and 1500 \ifmmode^\circ\else\textdegree\fi{}C. The maximum in ultrasonic wave travel times--corresponding to a minimum in acoustic velocities--shifts to higher pressure with increasing ${T}_{f}$ for both acoustic waves, with complete reversibility below 5 GPa. These relationships reflect polyamorphism in the supercooled liquid, which results in a glassy state possessing different proportions of domains of high- and low-density amorphous phases (HDA and LDA, respectively). The relative proportion of HDA and LDA is set at ${T}_{f}$ and remains fixed on compression below the permanent densification pressure. The bulk material exhibits compression behavior systematically dependent on synthesis conditions that arise from the presence of floppy modes in a mixture of HDA and LDA domains.