Compressibility of the high-pressure rocksalt phase of ZnO

We report the results of a combined experimental and theoretical investigation on the stability and the volume behavior under hydrostatic pressure of the rocksalt $(B1)$ phase of ZnO. Synchrotron-radiation x-ray powder-diffraction data are obtained from 0 to 30 GPa. Static simulations of the $\mathrm{ZnO} B1$ phase are performed using the ab initio perturbed ion method and the local and nonlocal approximations to the density-functional theory. After the pressure induced transition from the wurtzite phase, we have found that a large fraction of the $B1$ high-pressure phase is retained when pressure is released. The metastability of this ZnO polymorph is confirmed through the theoretical evaluation of the Hessian eigenvalues of a nine-parameter potential energy surface. This allows us to treat the experimental and theoretical pressure-volume data on an equal basis. In both cases, we have obtained values of the bulk modulus in the range of 160--194 GPa. For its zero-pressure first derivative, the experimental and theoretical data yield a value of $4.4\ifmmode\pm\else\textpm\fi{}1.0.$ Overall, our results show that the $\mathrm{ZnO} B1$ phase is slightly more compressible than previously reported.