Electronic properties of layered Ba0.5Sr0.5TiO3 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 SrTiO3 (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 Ba0.5Sr0.5TiO3 solid solution the gap is reduced by 0.2eV. The BST upper valence band consists of O2p 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.