Defect Clustering in Rare-Earth-Doped BaTiO3 and SrTiO3 and Its Influence on Dopant Incorporation

Rare-earth (RE) doping of BaTiO3 and SrTiO3 has received tremendous research attention in recent decades. Although Although RE doping has been a particularly popular topic, little or no attention has been given to the contribution from different defect arrangements. In this study, we use a proven interatomic potential model to calculate the binding energies of different RE donor defect (Ba/Sr site doping) configurations in BaTiO3 and SrTiO3. We consider two standard donor defect mechanisms, namely, charge compensation from Ba/Sr vacancies or Ti vacancies. Consistently stronger negative binding energies are calculated for SrTiO3 compared to BaTiO3. We also show that SrTiO3 can accept a larger range of RE ion sizes at the divalent cation site than BaTiO3. This is highlighted by the simple trend of increasing binding energy with decreasing RE ion size found for the defect configurations with a Ti vacancy in SrTiO3. Conversely, defect clustering in BaTiO3 can cause significant strain on the lattice, resulting in positive binding energies. Our results show that one standard defect arrangement per incorporation mechanism cannot be applied to all dopants. We clearly show that the lowest energy defect configuration is dependent on both the dopant and host material. The consequences of these findings for the overall defect incorporation energetics in BaTiO3 and SrTiO3 are also presented.