Electronic structures of hydrogen in perovskite-type oxide, SrTiO3

The electronic structures of hydrogen in SrTiO3 are simulated by the DV-X a lpha 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 agr eement with experiments. When Sc is doped into SrTiO3, an acceptor level ap pears just above the valence band. On the other hand, when hydrogen is intr oduced into SrTiO3, a donor level appears below the conduction band. The mo lecular 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 resul t of this charge compensation, the effective ionicity of hydrogen becomes a bout + 0.17 similar to + 0.24, the value of which is dependent on the hydro gen positions in the crystal lattice. Also, the chemical bond strengths bet ween constituent ions are modified largely by dopants. For example, the Sc doping tends to strengthen the chemical bond between hydrogen and oxygen io ns, 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.