Spectral dependences of the optical absorption in bismuth germanium oxide crystals annealed in vacuum

Results of experimental investigations into the spectral dependences of the light absorption coefficient of an undoped Bi ₁₂GeO ₂₀ crystal annealed in vacuum at a temperature of 650°S and subsequently annealed in the air atmosphere at temperatures in the range T _AA = 315-590°S are presented for the spectral range 400-1100 nm at room temperature. It is established that the optical absorption of the crystal after temperature annealing in vacuum increases in the entire examined spectral range. Subsequent annealing in air leads to gradual restoration of the initial impurity absorption spectrum in the range from 530 to 1100 nm. The wide band that can be caused by oxygen vacancies is observed in the spectrum of absorption changes induced by annealing in vacuum.