Pressure-induced softening of shear modes in ZnO

The room-temperature elastic moduli $C_{\mathrm{IJ}}$ corresponding to the pure longitudinal and transverse modes in single crystals of the wurtzite phase of ZnO have been determined from ultrasonic wave velocities measurements as a function of pressure up to 10 GPa. All the moduli exhibit a linear dependence on pressure, with positive values for the longitudinal moduli ${(dC}_{11}/dP=5.32\mathrm{}$ and ${\mathrm{dC}}_{33}/dP=3.78)$ but negative values for the shear moduli ${(dC}_{44}/dP=-0.35$ and ${\mathrm{dC}}_{66}/dP=-0.30).$ All modes exhibit anomalous travel time above $P_{\mathrm{Tr}}=7.5\mathrm{}\mathrm{GPa},$ indicating the onset of a transition to the rocksalt $\mathrm{}\left(B1\mathrm{}\right)\mathrm{}$ phase. Using recent improvements for ultrasonic measurements in multianvil apparatus, this experimental study on the phase-transformation mechanism is extended to simultaneous high pressures and high temperatures. At high temperatures, $P_{\mathrm{Tr}}$ decrease and the pressure derivatives of the elastic shear modes become more negative. Thus, the elastic shear softening observed at room temperature is enhanced at elevated temperatures.