  The thermodynamic and kinetic properties of mono and di-vacancy defects in
cubic (para-electric) barium titanate are studied by means of
density-functional theory calculations. It is determined which vacancy types
prevail for given thermodynamic boundary conditions. The calculations confirm
the established picture that vacancies occur in their nominal charge states
almost over the entire band gap. For the dominating range of the band gap the
di-vacancy binding energies are constant and negative. The system, therefore,
strives to achieve a state in which under metal-rich (oxygen-rich) conditions
all metal (oxygen) vacancies are bound in di-vacancy clusters. The migration
barriers are calculated for mono-vacancies in different charge states. Since
oxygen vacancies are found to readily migrate at typical growth temperatures,
di-vacancies can be formed at ease. The key results of the present study with
respect to the thermodynamic behavior of mono and di-vacancies influence the
initial defect distribution in the ferroelectric phases and therefore the
conditions for aging.
