Non-stoichiometric reconstructions on MgO(100)

The stability of non-stoichiometric reconstructed MgO{100} surfaces, with 1/4 and 1/2 densities of oxygen vacancies in the outermost layer, is studied using a total energy self-consistent calculation. All possible periodic configurations of vacancies are considered, up to a 70 Å2 area of the surface unit-cell. We demonstrate that the insulating configurations in which a neutralization of under-coordinated magnesiums takes place, as a result of a complete filling of their effective 3s levels, are associated with an enhanced surface stability. Among these configurations, we find that the most stable from an electrostatic point of view are those for which the neutral magnesiums have the largest number of vacancies in their first coordination shell. Consequently, the oxygen vacancies on MgO{100} are found to attract each other in an effective way and to present a tendency towards aggregation. This may explain why reconstructions have not been observed experimentally on non-stoichiometric MgO{100} surfaces.