First-principles study of A-site substitution in ferroelectric bismuth titanate

We employ density functional theory to investigate the effect of A-site rare earth substitution on the point defect formation in bismuth titanate (BIT) in the dilute substitution limit. Despite previous claims that the formation energy of neutral oxygen vacancies in La-substituted BIT (BLT) is higher than in pure BIT, our calculations show that this is only true for four out of the six distinct oxygen sites. Of these four sites, in two the difference is < 0.1 eV while in the other two the difference is similar to 0.25 eV. In the case of +2 charged oxygen vacancies, in only two of the six distinct oxygen sites is the formation energy of the oxygen vacancy higher in BLT than in BIT, where they differ by similar to 0.14 eV. These results do not support the traditional explanation for the fatigue-free characteristic of BLT, which states that La substitution might avoid the ferroelectric fatigue of BIT by simply suppressing the formation of oxygen vacancies.