Light-induced charge transfer processes in Mn-doped Bi12GeO20 and Bi12SiO20 single crystals

The optical absorption and ESR spectra of Bi₁₂GeO₂₀ and B₁₂SiO₂₀ doped with Mn have been measured before and after illumination with visible light. Uniaxial stress measurements on a sharp line observed at 8026 cm^?1 were performed. The observed ESR spectrum is a superposition of six lines resulting from the hyperfine interaction of manganese ions in tetrahedral positions. The g-factor and hyperfine constant are g = 1.999 ± 0.003 and A = 78 Gs. Analysis of the light-induced absorption spectrum leads to the conclusion that a small hole polaron bound to an Mn impurity at a tetrahedral site is responsible for the very broad absorption band which appears after illumination. The sharp line is interpreted as due to a transition inside the Mn⁺ center in tetrahedral coordination. Bands in the region 10,000–16,000 cm^?1 are due to Mn³⁺ centers in interstitial positions, whose symmetry can be treated to a first approximation as tetragonal. The following crystal field parameters for this center were found: B = 565 cm^?1, Dq = 1400 cm^?1, Dt = ?330 cm^?1, Ds = 4170 cm^?1 and C = 2260 cm^?1. The illumination conditions which are needed for homogeneous coloration of the sample are also discussed.