First principles study on oxygen vacancy formation in rock salt-type oxides MO (M: Mg, Ca, Sr and Ba)

Oxygen vacancy formation energies in rock salt-type oxides MO (M: Mg, Ca, Sr, and Ba) were investigated using a first-principles projector augmented wave method based on a density functional theory. Finite-size cell interactions were corrected by calculating with several sizes of the supercells up to 512 atoms, and a band gap correction for a neutral oxygen vacancy was also performed. It was commonly found that the vacancy state induced by the oxygen vacancy is placed below the conduction band minimum (CBM) in all compounds. The position of the vacancy state is deep in MgO and becomes closer to CBM in CaO, SrO, and BaO in that order. By analyzing the oxygen chemical potential dependence, it was found that the formation of oxygen vacancy is preferable under the reduction atmosphere in all compounds. In addition, it was also found that the oxygen vacancy formation energies in MgO, CaO, and SrO are similar to each other, whereas BaO shows lower formation energy than others, indicating that the oxygen vacancy is more abundant in BaO as compared with other rock salt-type oxides. The reason for the lower oxygen vacancy formation energy in BaO is discussed.