Tetrahedral Cr4+ and Cr5+ in Bi12TiO20 Single Crystals

Cr-doped Bi12TiO20 (BTO) single crystals have been grown by the Czochralski technique. The optical absorption at 4.2 K of as-grown crystals is compared to that observed after vacuum annealing of the samples. This treatment modifies the visible and near infrared optical absorption. The absorption spectrum of Cr-doped BTO crystals before vacuum annealing is interpreted as the superposition of the crystal-field transition bands of (CrO4)4— and (CrO4)3— and charge transfer bands of (CrO4)3—. Annealing in vacuum leads to Cr5+ → Cr4+ conversion. Chromium reduction to lower oxidation states has not been observed. The optical absorption shows a vibrational character. The energy shifts of Cr4+ bands are as expected from the coupling with the T2(ν3) fundamental mode of (CrO4)4— clusters. On the other hand, it is proposed that the series of sharp lines observed in the 9500 to 11500 cm−;1 range arises from the interaction with a vibration of about 300 cm−;1, which seems to be the T2(ν4) fundamental mode of (CrO4)3— and with the lattice modes.