Elastic wave velocities of silica glass at high temperatures and high pressures

The elastic compressional (P) and shear (S) wave velocities of silica glass at simultaneous high pressures and high temperatures were measured using a combination of ultrasonic interferometry, in situ synchrotron x-ray diffraction, and radiographic techniques in a large-volume Kawai-type multianvil apparatus. We found no first-order amorphous-amorphous transformation throughout the pressure and temperature conditions between 2 and 5 GPa and up to 1173 K, contrary to previous studies suggested by piston cylinder experiments or molecular dynamics simulations. For each of the 650 and 800 K isothermal runs, the P and S wave velocities initially decreased with increasing pressure, reaching minimum values at around 3–4 GPa, followed by increases with pressure up to 6.1 GPa; on successive decompression to ambient pressure, both velocities changed irreversibly due to permanent densification, and no minima were observed in both velocities. We also found that, in a second compression-decompression cycle at 800 K, the densified silica glass was compressed reversibly (elastically) within errors without further irreversible densification. Using the measured P and S wave velocities in such reversible (elastic) compression regions as a function of pressure, we found the density of silica glass increases with temperature from 300 to 800 K at all the measured pressure range up to 6.1 GPa, providing a direct evidence of a negative thermal expansion of silica glass at high pressures.