Nominally undoped YAl${\mathrm{O}}_3$ shows broad intense absorption bands in the visible after irradiation with visible and near uv light at 77 K; they become especially strong after subsequent irradiation with near ir. The bands could be correlated with an ESR spectrum arising from holes, which are trapped at ${\mathrm{O}}^{2-}$ ions near unidentified defects or are possibly self-trapped. A similar spectrum appeared after heating to 200 K. It was rather weak and a corresponding absorption could not definitely be identified. The optical absorption is explained by treating the holes together with their accompanying lattice distortions as small polarons. A theory based on this model is presented which is a generalization of an earlier treatment of bound-small-polaron absorption. Information on further absorption of the crystals up to 4 eV is given. Most of this absorption is related to the formation or quenching of the polaron bands.

• Received 18 November 1974

DOI:https://doi.org/10.1103/PhysRevB.11.4201