First principles calculations of niobium substitution in strontium titanate

We have studied the effects of Nb incorporation in strontium titanate using DFT plane-wave pseudopotential calculations. Substitution of the impurity on a Ti site in the bulk crystal causes outward relaxations of the neighbouring Ti ions but does not affect the oxygen ions. A conduction band state localized on the Ti ions becomes occupied confirming the donor behaviour of the defect. The formation energy of the impurity is studied under different oxidation conditions. We have also studied the incorporation of Nb near to a Sigma = 3 (111) grain boundary. The results indicate that Nb segregation is unfavourable due to Coulomb repulsion effects.