Abstract
Optical absorption and ESR spectra of Bi 12GeO 20 doped with Cr were measured before and after illumination with visible light. It was found that Cr 4+ ions in tetrahedral position are responsible for light induced ESR and optical spectra. The g-factor of the center is 1.945 ± 0.002. Crystal field parameters for the Cr 4+ center are D q = 820 cm −1 and B = 429cm −1. The photochromic effect is explained in terms of a Cr 5+−Cr 4+ charge transfer process.
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