Electronic structures of hydrogen in perovskite-type oxide, SrTiO3

The electronic structures of hydrogen in SrTiO3 are simulated by the DV-X alpha molecular orbital method. In pure SrTiO3, there is a band gap of about 3.5 eV between the O-2p valence band and the Ti-3d conduction band, in agreement with experiments. When Sc is doped into SrTiO3, an acceptor level appears just above the valence band. On the other hand, when hydrogen is introduced into SrTiO3, a donor level appears below the conduction band. The molecular orbital of the donor level is composed mostly of the Ti-3d and 0-2p electrons, but still there is a small occupancy (6% similar to 12%) of the H-ls electrons in it. When both Sc and hydrogen coexist in SrTiO3, charge transfer takes place from the donor level to the acceptor level. As a result of this charge compensation, the effective ionicity of hydrogen becomes about + 0.17 similar to + 0.24, the value of which is dependent on the hydrogen positions in the crystal lattice. Also, the chemical bond strengths between constituent ions are modified largely by dopants. For example, the Sc doping tends to strengthen the chemical bond between hydrogen and oxygen ions, but instead to weaken the chemical bond between the oxygen ion and the surrounding metal ions. In addition, it is shown that an oxygen ion vacancy makes the defect level below the conduction band in the Sc-doped oxide. (C) 1999 Published by Elsevier Science B.V. All rights reserved.