Role of surface heterogeneity in the chemical bond of MgO: ionic character of regular and defect surface sites

Ab initio cluster model wavefunctions for bulk, surface and defect sites of crystalline MgO have been obtained in order to determine possible variations in the degree of ionicity in different physical environments. The degree of ionicity has been estimated from two different and complementary theoretical techniques. On the one hand, the constrained space orbital variation method allows us to quantify the importance of the different energetic contributions to the total energy. Also, projection techniques applied to the final Hartree-Fock wavefunction, permit us to determine the importance of covalent effects. Our analyses show that MgO behaves as an almost perfect ionic crystal and that there is no significant increase in the covalent contributions on the regular surface sites. On the other hand, less coordinated sites like steps and corners, exhibit a moderate increase of covalency. This behaviour is rationalized in terms of a reduced Madelung field at the defect sites.