Abstract
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 ${(dC}_{11}/dP=5.32$ and ${\mathrm{dC}}_{33}/dP=3.78)$ but negative values for the shear moduli ${(dC}_{44}/dP=\ensuremath{-}0.35$ and ${\mathrm{dC}}_{66}/dP=\ensuremath{-}0.30).$ All modes exhibit anomalous travel time above ${P}_{\mathrm{Tr}}=7.5\mathrm{GPa},$ indicating the onset of a transition to the rocksalt $(B1)$ 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.
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