DETERMINATION OF THE FACTORS CONTROLLING THE OPTICAL BACKGROUND ABSORPTION IN NOMINALLY UNDOPED AND DOPED SILLENITES

We investigated the background absorption spectrum of undoped and dope d crystals within the BMO family (Bi12MO20, M = Ge, Si, Ti). Dopants w ere Al, Fe, Cu, Cu/V, and Cr. Many of the observed effects are consist ent with the antisite Bi-M absorption centre model and the derived pre dictions. The absorption level primarily depends on the presence of ne utral BiM centres. Their density in undoped material is determined by the nature of M. The effect of added chemical impurities is either to reduce or to enlarge the absorption coefficient. Concerning the influe nce of dopants, important factors are the valence state, their ability to take different valences, their concentration, their solubility, th e symmetry of the occupied site, and magnitude and spectral dependence of their absorption cross section. Doping of EGO with Cu and V highly shifts the optical absorption towards the red and IR region. (C) 1998 Elsevier Science B.V. All rights reserved.