Electronic structure of SrTi1-xMxO3-? (M = Co, Ni, Cu) perovskite-type doped-titanate crystals by DFT and DFT + U calculations

We study the electronic structure of Co, Ni, and Cu impurities and of their vacancy-associated complexes in SrTiO3 crystals by means of density functional theory (DFT) and DFT + U calculations. Simple substitutionals prefer low-spin states, as expected from ligand field theory. On the other hand, the formation energy of vacancies (which tend to stay close to the impurity) is rather low for all the doped systems, and decreases with Z. This reflects the low stability of the +IV oxidation state for the impurities. Contrary to simple impurities, high-spin states are predicted to be stable when impurities are associated to vacancies.