Correlation between the impurity content, the average charge state of chromium cations and optical absorption in sillenite crystals doped by chromiumin wide range of concentrations

Single crystals of sillenites (Bi12SiO20, Bi12TiO20) doped with chromium in a wide range of concentrations from 1 x 10(-5) up to 1.8 X 10(-2) wt. % we re grown by the Czochralski and top-seed solution growth (TSSG) techniques. To estimate the content and the average charge state of chromium in the gr own crystals the chemical analysis by the modified diphenylcarbazide method was applied. The dependencies of the chromium distribution coefficient, th e average charge state of Cr cations, and optical absorption on the Cr conc entration were found. Both the shift of the absorption edge toward lower fr equencies and the appearance of an additional absorption band in the near I R were observed when the chromium concentration in the crystals was increas ed. The experimental data suggest that chromium has at least two charge sta tes and occupies probably different positions in the sillenite unit cell.