O- holes associated with alkali acceptors in BaTiO3

Abstract

In BaTiO3 the alkali ions Na and K, replacing Ba, lead to acceptor levels about 50 meV above the valence band edge. The structure of these defects can be studied by electron spin resonance, after light induced valence-band holes have been captured at neighboring O2- ions. Hyperfine interaction with Na, Ba, and Sr impurities, adjacent to the paramagnetic O-, is-identified. The holes are trapped stably below similar to 50 K. Their level position is determined by the temperature dependence of the relaxation rate of the light-induced nonequilibrium excess hole population. Photostimulated small polaron transfer between O2- sites equivalent with respect to Na or K leads to a strong optical-absorption band with peak at 1.3 eV, corresponding to a stabilization energy of the bound hole polaron of about 0.65 eV. The Na+-O- dipoles can be reoriented under uniaxial stress at 4.2 K; a differential stress coupling coefficient of 1.23x10(-4) m(3) is derived. Several O- centers associated with partly unassigned accepters on Ti sites are identified. Their properties are compared to those of the Ba site accepters. Vibronic g shifts are accounted for by a dynamic Jahn-Teller effect involving the twofold degenerate O- pi orbitals.