High-density phases of ZnO: Structural and compressive parameters

Dense ZnO phases, obtained by static compression at room temperature, are studied by x-ray diffraction using synchrotron radiation. The initial wurtzite structure of ZnO is observed to transform to the rocksalt structure at $9.1\pm 0.2\mathrm{GPa}$ with a large volume collapse of 16.7% on increasing pressure. Accurately determined structural and compressive parameters over a large pressure range for both phases are presented. We report the following values for the null compression bulk moduli and their pressure derivatives: 142.6 GPa and 3.6 and 202.5 GPa and 3.54, for the wurtzite and the rocksalt phases, respectively. From a comparison between the present results and calculated ones, it is inferred that Hartree-Fock calculations yield the best estimation of the compressive parameters in dense ZnO. Furthermore, we show that the ideal relationship between the $c/a$ axial ratio and the $u$ oxygen positional parameter is preserved as the wurtzite lattice becomes denser while approaching the structural transition pressure.