TETRAHEDRAL CR4-CRYSTALS( AND CR5+ IN BI12TIO20 SINGLE)

Cr-doped Bi12TiO20 (BTO) single crystals have been grown by the Czochr alski technique. The optical absorption at 4.2 K of as-grown crystals is compared to that observed after vacuum annealing of the samples. Th is treatment modifies the visible and near infrared optical absorption . The absorption spectrum of Cr-doped BTO crystals before vacuum annea ling is interpreted as the superposition of the crystal-field transiti on bands of (CrO4)(4-) and (CrO4)(3-) and charge transfer bands of (Cr O4)(3-). Annealing in vacuum leads to Cr5+ --> Cr4+ conversion. Chromi um reduction to lower oxidation states has not been observed. The opti cal absorption shows a vibrational character. The energy shifts of Cr4 + bands are as expected from the coupling with the T-2(v(3)) fundament al mode of (CrO4)(4-) clusters. On the other hand, it is proposed that the series of sharp lines observed in the 9500 and 11500 cm(-1) range arises from the interaction with a vibration of about 300 cm(-1), whi ch seems to be the T-2(v(4)) fundamental mode of (CrO4)(3-) and with t he lattice modes.