Ba_cSr_1-cTiO₃ perovskite solid solutions: Thermodynamics from ab initio electronic structure calculations

We suggest theoretical prediction for Ba_cSr _1-cTiO₃ perovskite solid solutions (BST) combining ab initio DFT/B3PW calculations and alloy thermodynamics. This approach is based on calculations of a series of ordered super-structures in Ba-Sr simple cubic sublattice immersed in the rest TiO₃ matrix. Although these structures are unstable with respect to the decomposition, the results of total energy calculations allow us to extract the necessary energy parameters and to calculate the phase diagram for the solid solutions (alloys). A novel approach applied to the BST system enables to predict that at T > 400 K Ba and Sr atom distribution is random. But below this temperature at small c Ba atoms aggregate into nanoclusters, thus leading to the formation of Ba-rich complexes of "almost pure" BaTiO₃ (BTO) in mostly SrTiO₃ (STO) matrix. At large c the formation of analogous SrTiO₃ complexes in BaTiO₃ is predicted.