NONSTOICHIOMETRIC RECONSTRUCTIONS ON MGO(100)

The stability of non-stoichiometric reconstructed MgO{100} surfaces, w ith 1/4 and 1/2 densities of oxygen vacancies in the outermost layer, is studied using a total energy self-consistent calculation. All possi ble periodic configurations of vacancies are considered, up to a 70 An gstrom(2) area of the surface unit-cell. We demonstrate that the insul ating configurations in which a neutralization of under-coordinated ma gnesiums takes place, as a result of a complete filling of their effec tive 3s levels, are associated with an enhanced surface stability. Amo ng these configurations, we find that the most stable from an electros tatic point of view are those for which the neutral magnesiums have th e largest number of vacancies in their first coordination shell. Conse quently, the oxygen vacancies on MgO{100} are found to attract each ot her in an effective way and to present a tendency towards aggregation. This may explain why reconstructions have not been observed experimen tally on non-stoichiometric MgO{100} surfaces.