Stabilization of the (1 1 1) surface of NiO and CoO by segregation of point defects

Vacancies are ubiquitous point defects in transition metal oxides. Surprisingly, the effect of vacancies on the structural stability of oxide surfaces has been neglected in the last decades. On the other hand, experimentally observed surface structures of oxides are frequently in contrast to theoretical predictions. A typical example is the widely observed (1 1 1) surface of the rock-salt-type NiO and CoO, which is predicted to be polar and unstable according to electrostatic arguments. Previous theoretical models proposed to compensate the surface polarity assume a perfect bulk, despite the high density of cation vacancies in these materials. Through first-principles calculations, we show in this work that the cation vacancies have a strong tendency to segregate to the surface, leading to the formation of surfaces with high-proportion of cation vacancies even at low oxygen potentials. Energetically, the segregated surfaces are more stable than the well-known octopolar reconstruction. The role of segregation of point defects in surface behaviors of transition metal oxides may be of significance for understanding their catalysis and electronic properties.