The segregation of Na_(Sr) impurities to a Σ = 5 [001] twist boundary in SrTiO_3 is studied using DFT-based plane-wave pseudopotential techniques. The formation energies of the impurities are calculated as a function of oxygen chemical potential and electron chemical potential. The results indicate a strong driving force for segregation to the boundary and that the Na impurities exhibit acceptor-like behaviour. The atomic displacements caused by the impurities are small both in the bulk and at the grain boundary. Based on the results a model is suggested in which Na_(Sr) segregation is driven by soft relaxation of the electronic structure.
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