Rietveld refinement study of the pressure dependence of the internal structural parameter u in the wurtzite phase of ZnO

The pressure dependence of the internal structural parameter u of the wurtzite phase of ZnO was studied using high resolution angular dispersive x-ray diffraction up to 12 GPa in a helium pressure medium. The u parameter increases with pressure up to about 0.430 at about 5.6 GPa, and then rapidly decrease to about 0.380 during the phase transition. Two models of the phase transition path were proposed recently by first-principles calculations [ A. M. Saitta and F. Decremps Phys. Rev. B 70 035214 (2004); S. Limpijumnong and S. Jungthawan Phys. Rev. B 70 054104 (2004)]. These possible mechanisms responsible for the wurtzite-to-rocksalt phase transition under high pressure are experimentally evaluated based on our results. The bulk moduli of the wurtzite and rocksalt phase at zero pressure were estimated as 135.3±1.8 and 177.4±4.6 GPa, respectively.