Absorption tail and fundamental absorption edge in vanadium phosphate glasses

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Abstract
Optical measurements of the fundamental absorption edge and broad absorption tail of 50 mole% V2O5 vanadium phosphate glass films were made in the range 3850–30 000 cm−1 (2.6–0.33 μ) before and after reducing and oxidizing anneals. The absorption in the tail region of the fundamental absorption increased during reducing anneals and decreased during oxidizing anneals. A band at 19400 cm−1 (2.41 eV) was introduced by the reducing anneals. The band gap Eg was determined to be 2.71, 2.92, and 2.82 eV after successive anneals in oxygen, hydrogen, and oxygen, respectively. Evidence for direct forbidden transitions was observed from fundamental absorption edge measurements similar to that for crystalline V2O5. The concentration of reduced V4+ ions is concluded to be largely responsible for the variation of Eg and of the absorption tail.