The optical properties of nominally pure crystals as well as of those of pure and and -doped powders are compared. The emission band peaking at 3.9 eV and the excitation bands in the region 10-10.6 eV observed for and powders are ascribed to the radiative decay of a self-trapped exciton perturbed by an anion vacancy or impurity-vacancy dipole. A -perturbed exciton has been found in .