Theoretical evidence for unexpected O-rich phases at corners of MgO surfaces

Introducing charge carriers into MgO via $p$ doping greatly reduces formation energy of an O-vacancy in the bulk and at the (100) surface $[1]$. In this work, we use hybrid density functional theory to explore O-vacancy and O/O$_2$-ad-species defects at corners of MgO surfaces. The defects are modelled using MgO clusters embedded into a field of norm-conserving pseudopotentials and point charges. The long-range response of the oxide to the charge carriers trapped at the defects is taken into account using a polarizable force field. The low-energy defect atomic structures are found using an {\em ab initio} genetic algorithm $[2]$. Concentrations of O-vacancies and O-ad-species at realistic temperatures and pressures are obtained with {\em ab initio} atomistic thermodynamics. Unexpectedly, we find that O-ad-species rather than O-vacancies are dominating defects at realistic conditions. The stability of the O-ad-species over O-vacancies and pristine corners is explained by an interplay between bond-breaking, bond-making, and charge-carrier trapping. $-$ $[1]$ N. Richter {\em et al.}, Phys. Rev. Lett. {\bf 111}, 045502 (2013); $[2]$ S. Bhattacharya {\em et al.}, New J. Phys., in press (2014)