A GENERIC IONIC POTENTIAL FOR THE ALKALINE-EARTH OXIDES AND THE ANOMALOUS CRYSTAL-STRUCTURE OF ZNO

It is shown that 'compressible ion' potentials, which represent the re sults of ab initio electronic structure calculations on MgO and CaO an d which represent their phase transitions accurately, can be transferr ed successfully to other alkaline earth oxides by substitution of acce pted values for the cation radii. The application of such a potential to ZnO erroneously predicts a ground-state crystal structure of rocksa lt symmetry. After extending the interaction model to allow for the pr esence of induced octupoles on both the zinc and oxide ions the observ ed wurtzite structure emerges as most stable. The importance of short- range contributions to the induced moments, beyond those predicted by the multipole expansion, is highlighted.