Impurity defects in Cr-doped Bi12SiO20 and Bi12TiO20 crystals

Bi12SiO20 and Bi12TiO20 crystals doped with 10(-5) to 2 x 10(-2) wt % Cr we re grown by the Czochralski technique. The Ct content of the crystals and t he valence state of Cr ions were determined by a new analytical technique w ith the use of 1,5-diphenylcarbazide. Correlations were revealed between th e segregation coefficient of the dopant, the charge state of Cr ions, and t he absorption coefficient of the crystals. Doping with Cr was found to shif t the intrinsic edge to longer wavelengths and to give rise to a new absorp tion band in the near-IR region. The results are interpreted as indicating that the Cr ions in the sillenite crystals studied are in at least two oxid ation states and occupy inequivalent sites in the sillenite cell.