Density functional study of the polar MnO(111) surface

By application of a density functional approach within the PBE and PBE+U approximations we investigate the ground state terminations of the polar MnO(111) surface being in thermodynamic equilibrium with an oxygen reservoir. In the allowed range of the oxygen chemical potential and for realistic oxygen partial pressures the surface is found to undergo different structural transitions. In the oxygen-poor regime the most stable phases are the O- and Mn-terminated octopolar structures, which are almost degenerate in energy. For oxygen-rich conditions we observe a competition between the O-terminated unreconstructed bulk face and a stripes structure. We show that the stabilization of the polar surface in the thermodynamic equilibrium with the oxygen environment is due to remarkable changes of the geometrical structure (i.e., reconstruction and relaxation) and of the electronic structure (i.e., metallization).