LDA and GGA calculations for high-pressure phase transitions in ZnO and MgO

We report total energy and electronic structure calculations for ZnO in the B4 (wurtzite), B3 (zinc blende), B1 (rocksalt) and B2 (CsCl) crystal structures over a range of unit cell volumes. We employed both the local density approximation (LDA) and the PBE96 form of the generalized gradient approximation (GGA) together with optimized Gaussian basis sets to expand the crystal orbitals and periodic electron density. In agreement with earlier ab initio calculations and with experiment, we find that the B4 phase of ZnO is slightly lower in energy than the B3 phase, and that it transforms first to the B1 structure under applied pressure. The equilibrium transition pressure pT1 is 6.6 GPa at the LDA level of theory and 9.3 GPa in the GGA, compared to experimental values around 9 GPa. This confirms a trend seen by other workers in which the LDA underestimates structural transition pressures which are more accurately predicted by the GGA. However, some aspects of the equations of state are better predicted by the LDA, including the bulk moduli. At much higher compression, we predict that the B1 phase of ZnO will transform to the B2 (cesium chloride) structure at pT2 = 260 GPa (LDA) or 256 GPa (GGA) indicating that gradient corrections are unimportant for pressures in the megabar range. This is the first quantitative prediction of this transition in ZnO, and should be testable with diamond anvil techniques. We predict that ZnO remains a semiconductor up to pT2. For comparison we find that the B1 to B2 transition in MgO occurs at 515 GPa with either LDA or GGA, in excellent agreement with other ab initio predictions.