O- HOLES ASSOCIATED WITH ALKALI ACCEPTORS IN BATIO3

In BaTiO3 the alkali ions Na and K, replacing Ba, lead to acceptor lev els about 50 meV above the valence band edge. The structure of these d efects can be studied by electron spin resonance, after light induced valence-band holes have been captured at neighboring O2- ions. Hyperfi ne interaction with Na, Ba, and Sr impurities, adjacent to the paramag netic O-, is-identified. The holes are trapped stably below similar to 50 K. Their level position is determined by the temperature dependenc e of the relaxation rate of the light-induced nonequilibrium excess ho le population. Photostimulated small polaron transfer between O2- site s equivalent with respect to Na or K leads to a strong optical-absorpt ion 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 coupl ing coefficient of 1.23x10(-4) m(3) is derived. Several O- centers ass ociated with partly unassigned accepters on Ti sites are identified. T heir properties are compared to those of the Ba site accepters. Vibron ic g shifts are accounted for by a dynamic Jahn-Teller effect involvin g the twofold degenerate O- pi orbitals.