Optical Reflection Spectra of Chromia—Alumina

The optical reflection spectra from 12 500 to 50 000 cm—1 were studied for a series of coprecipitated chromia—alumina samples calcined between 500° and 1400°C. The Melamed theory was employed to obtain extinction coefficients from these spectra. The Racah parameter B and the wavenumbers of the two visible transitions in ruby (1400°C calcination) arising from Cr3+ gradually decreased with increasing chromia content. The Racah parameter B and the wavenumber Δ of the 4A2→4T2 transition in high chromia content samples calcined at 500°, 750°, and 900°C. were unusually low due to the formation of chromia rich solid solutions. In the oxidized state the samples calcined below 1400°C showed an intense ultraviolet absorption due to Cr6+.