Impurity and vacancy clustering at the Σ3(111)[1−10] grain boundary in strontium titanate

Abstract

The interaction of selected point defects and the Σ3(1 1 1)[1 −1 0] symmetrical twin boundary in SrTiO3 is investigated by density-functional band-structure calculations. The pristine boundary is SrO3-terminated and exhibits electronic properties which are comparable with the pure bulk phase. Both the mirror-symmetric twin and some laterally shifted structures have low grain boundary energies, thus they may coexist in a real crystal if external stress is applied. With varying chemical potential of the electrons, the TiO3 sublattice of the pristine boundary is destabilised by both electron enrichment and electron depletion. These results may explain, why different translation states can be observed by electron microscopy within the same bicrystal. Substitutional doping of the Ti columns next to the boundary plane with formally 3+ cations yields a contraction compared with the pure boundary. The same result, accompanied by more pronounced local geometry changes, is obtained for the translation state in the presence of O vacancies. Neutral O vacancies in the boundary plane are the most stable species followed by positively charged O vacancies in the neighbouring Sr/O plane parallel to the boundary. The Fermi level is shifted either upward to metal-derived states by substitutional doping or downward into the O-2p manifold for O vacancies. These findings may rationalise both the observed low, mainly electronic, grain boundary conductivity of SrTiO3 boundaries and the changes in local potential found at the grain boundary by tunnelling and force microscopy experiments.