Optical Absorption Properties of Vanadate Glasses

The optical absorption of vanadate glasses based on the system V₂O₅–P₂O₅ was measured in the range 20 cm⁻¹ to 25 000 cm⁻¹ at room and liquid‐nitrogen temperatures. The samples were blown films of thickness about 1–2 μ and of composition 70.0, 80.0, and 87.5 mole % V₂O₅. Vibrational absorption peaks were observed at about 360, 420, 680, 1010, and 1100 cm⁻¹ with additional structure likely at about 900 cm⁻¹ in the 70.0 mole% V₂O₅ films. Peaks were observed at about 330, 435, 635, 810, 1007, and 1085 cm⁻¹ in the 87.5 mole % films. Absorption tails were observed extending from the lowest‐energy peaks to 20 and 33 cm⁻¹ in the 70.0 and 87.5 mole % V₂O₅ samples, respectively. No noticeable temperature effects on spectra shape and peak positions were observed. Absorption peaks were also observed at 1038 and 1277 cm⁻¹ in crystalline V₂O₅ at room temperature and at 915, 1040, and 1274 cm⁻¹ and possibly at 1256 cm⁻¹ at liquid‐nitrogen temperature. The peaks at about 1010 cm⁻¹ in the glasses are thought to be the V–O stretching vibrations, and the peaks at about 1090 cm⁻¹ are assigned to a phosphorous–oxygen vibration. Other peaks are unassigned. A broad absorption tail which is responsible for the dark black color of bulk samples was observed between the apparent fundamental absorption edge of the glasses in the short‐wavelength region of the visible and about 4000 cm⁻¹. The cause of this absorption is not definitely ascertained, though V⁴⁺ ions may contribute. The absorption edge of both the 70.0 and 87.5 mole % V₂O₅ glasses fits the condition for direct forbidden transitions as does the edge of crystalline V₂O₅. $E_g$ values were determined to be 2.38 and 2.41 eV for 87.5 mole % V₂O₅ films at room and liquid‐nitrogen temperatures, respectively, and 2.47 and 2.51 eV for the 70.0 mole % V₂O₅ films at room and liquid‐nitrogen temperatures, respectively.