High-temperature phase transitions and elasticity of silica polymorphs

The single-crystal acoustic velocities of α- and β-quartz were measured by Brillouin spectroscopy to a maximum temperature >1,500°C at room pressure. From these velocities, the single-crystal elastic moduli were calculated up to 1,050°C, exceeding the temperature range of previous measurements by 350°C for the elastic moduli and by 710°C for acoustic velocities. The ordinary refractive index (n o) of α- and β-quartz was measured from room temperature to 800°C. In the temperature interval from ∼950 to 1,000°C a subtle change in the temperature derivative of the longitudinal acoustic velocity was observed in platelet geometry for all measured directions. The high-temperature acoustic velocity data may indicate the presence of a second phase, presumably β-cristobalite, that nucleates below 1,000°C.