Pressure-induced softening of shear modes in ZnO

The room-temperature elastic moduli C_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 (dC₁₁/dP = 5.32 and dC₃₃/dP = 3.78) but negative values for the shear moduli (dC₄₄/dP = -0.35 and dC₆₆/dP = -0.30). All modes exhibit anomalous travel time above P_Tr = 7.5 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_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.