It was recalled that, when Na and K replaced Ba in this material, acceptor levels were produced that were about 0.05eV above the valence band edge. The structure of these defects could be studied by using electron spin resonance methods, after light-induced valence-band holes were captured at neighboring O2- ions. Hyperfine interaction with Na, Ba and Sr impurities which were adjacent to the paramagnetic O- was identified. The holes were stably trapped at temperatures below about 50K. Their level-position was governed by the temperature dependence of the relaxation rate of the light-induced non-equilibrium excess hole population. Photo-stimulated small-polaron transfer between O2- sites that were equivalent with respect to Na or K led to a strong optical absorption band, with a peak at 1.3eV, that corresponded to a stabilization energy of the bound hole polaron which was equal to about 0.65eV. The Na+-O- dipoles could be reoriented by a uniaxial stress at 4.2K. A differential stress coupling coefficient of 0.000123m3 was derived. Several O- centers which were associated with partly unassigned acceptors on Ti sites were identified. Their properties were compared with those of the Ba site acceptors. The vibronic g-shifts were accounted for in terms of a dynamic Jahn-Teller effect which involved the 2-fold degenerate O- -orbitals.

T.Varnhorst, O.E.Schirmer, H.Kröse, R.Scharfschwerdt, T.W.Kool: Physical Review B, 1996, 53[1], 116-25