SPECTROSCOPIC AND CHIROPTICAL PROPERTIES OF DOPED SILLENITE CRYSTALS .6. EFFECTS OF ILLUMINATION AND ANNEALING ON THE PROPERTIES OF CR-DOPED BI12TIO20 AND BI12SIO20 CRYSTALS

Cr-doped Bi12TiO20 and Bi12SiO20 crystals were grown by the Czochralsk i technique. We studied the effects of illumination and successive ann eals in vacuum and air on the absorption, circular dichroism, diffuse reflectance, and IR spectra of Bi12TiO20/[Cr] and Bi12SiO20[Cr] crysta ls and chromium sillenite powders. Chromium was found to occupy tetrah edral sites of the sillenite structure in the oxidation states 5+ and 4+. In as-grown crystals, the Ci(5+)/Cr4+ ratio is greater than unity and remains unchanged over the entire concentration range studied. Pho toexcitation and vacuum annealing of Bi12TiO20/[Cr] and Bi12SiO20[Cr] crystals results in the Cr5+ --> Cr4+ reduction, but further reduction of Cr4+ is not observed. Upon air annealing, the Cr5+/Cr4+ ratio retu rns to its initial value.