O- bound polarons in oxide materials

Abstract

The optical absorption of holes, self-trapped at one of the equivalent O2- sites next to acceptor defects in crystalline oxide materials, is treated. The holes are localized by lattice distortion, forming small polarons with typical stabilization energies EP of 1 eV. The light-induced hole transfer from an initial O- site to an equivalent O2- neighbour occurs under Franck-Condon conditions, leading to the widest possible homogeneous absorption bands, having high oscillator strength, typically 0.1, and peak energies between 8/3 EP and 1 EP, for 4 up to 12 equivalent neighbours. The basic absorption features, including characteristic excited state tunnelling splittings, are exemplified with representative defects of high point symmetry. Such polarons arise in numerous materials after excitation by light or ionizing radiation. They play important roles as sources of coloration in non-linear optic and photorefractive compounds, gemstones, as acceptor defects in oxide semiconductors and as catalytic agents, if at a surface. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA.