HARTREE-FOCK STUDY OF PHASE-CHANGES IN ZNO AT HIGH-PRESSURE

The total energy of ZnO as a function of unit cell volume has been cal culated for the zinc-blende, wurtzite, and rocksalt structures by the ab initio all-electron periodic Hartree-Fock linear-combination-of-ato mic-orbitals method using a large Gaussian basis set that was variatio nally optimized for the solid state. Extensive convergence tests with respect to cutoffs of the real-space Coulomb and exchange series were carried out to ensure that the calculations were performed with suffic ient precision. The calculated structural properties (equilibrium latt ice constant, bulk modulus, etc.) of the wurtzite and rocksalt phases are in good agreement with experiment, as is the transition pressure b etween them (8.57 GPa versus 9-9.5 GPa experimentally), indicating tha t the method can reliably predict quite small energy differences betwe en different densities or crystal structures of a nonmetallic solid. T he calculated valence-band structure was also in satisfactory agreemen t with experiment. Detailed analysis of the charge-density distributio n supports the expected picture of a transition from mixed ionic-coval ent bonding in the tetrahedrally coordinated structures to predominant ly ionic bonding in the high-pressure phase.