Ab initio study of vacancies in cubic BN under pressure

We investigate, through first-principles calculations, the pressure dependence of formation volumes and formation enthalpies of boron and nitrogen vacancies (VB, VN) in cubic boron nitride (cc-BN) using a supercell approach. We find that VB3− and VN3+ have the lowest formation enthalpy and −3 and +3 can be considered as the dominant charges occurring in VB and VN at ambient pressure, respectively. And the charge states which have the lowest formation enthalpies do not change with pressure in the pressure range from 0 to 20 GPa. The formation enthalpy decreases with pressure for VB, while for VN it exhibits positive dependence of the formation enthalpy on pressure. Energy levels of defects under different pressures are also discussed. Our results suggest that for VN, pressure can strengthen its conductivity, while for VB, pressure effect is not obvious in our investigated pressure range.