Electronic properties of layered Ba_0.5Sr_0.5TiO ₃ heterostructure: Ab initio hybrid density functional calculations

Ab initio calculations of the BST heterostructure with equiatomic constituent of Sr and Ba species has been carried out within hybrid functional B3PW involving a hybrid of non-local Fock exchange and Becke's gradient corrected exchange functional combined with the non-local gradient corrected correlation potential by Perdew and Wang. The suggested scheme of calculations reproduces experimental lattice parameters of both pure BaTiO₃ and SrTiO₃. The calculated optical band gap for the pure SrTiO ₃ (BaTiO₃) is 3.56 (3.46)eV (expt. 3.25 and 3.20eV, respectively), i.e. agreement is much better than in the standard LDA or HF calculations. In the Ba_0.5Sr_0.5TiO₃ solid solution the gap is reduced by 0.2eV. The BST upper valence band consists of O 2p atomic orbitals (AO) with a small admixture of Ti 3d, whereas the conduction band bottom consists mostly of Ti 3d orbitals with a small contribution of O orbitals. The Sr (Ba) atomic orbitals make a significant contribution to the higher part of the conduction band. 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.