The oxygen vacancy in BiFeO3 was calculated to be a double donor with states

0.6eV below the conduction band edge, consistent with cathodoluminescence and

electronic conductivity data. The atomic configurations were relaxed using the

local density approximation plus Hubbard U (LDA+U) to the electron-correlation

energy for each defect charge state to ensure that the oxide had a non-zero bandgap.

The defect formation energies were calculated using the screened exchange

functional.

Energy Levels of Oxygen Vacancies in BiFeO3 by Screened Exchange. S.J.Clark,

J.Robertson: Applied Physics Letters, 2009, 94[2], 022902