Ab initio calculations of the atomic and electronic structure of layered Ba_0.5Sr_0.5TiO₃ structures

Understanding of the atomic and electronic structure of Ba _cSr_{1 - c}TiO₃ (BST) solid solutions is important for several applications including the non-volatile ferroelectric memories (dynamic random access memory, DRAM). We present results of ab initio calculations of several spatial arrangements of Ba_0.5Sr _0.5TiO₃ solid solutions based on DFT-HF B3PW hybrid method. We calculate the atomic and electronic structure, the effective charges, interatomic bond populations, the electronic density distribution, and densities of states for three layered structures with the same composition. The suggested method reproduces experimental lattice parameters of both pure BaTiO₃ and SrTiO₃. The calculated optical band gaps for the pure SrTiO₃ and BaTiO₃ are in a good agreement with experimental data, much better than those from the standard LDA or HF calculations. In the studied BST structures with the equiatomic composition (c = 0.5) the gap is reduced by ca. 0.2 eV. The electron density maps demonstrate the covalency effects in the Ti-O bonding. The electron density near the Sr atoms is stronger localized, as compared with the Ba ions.