Optical properties of chromium-doped transparent alumina xerogels

Optical properties of chromium-doped transparent alumina xerogels have been studied in the ultraviolet, visible and near infra-red regions. The coordination, valence state and symmetry of crystal field around the transition metal ion is found to change in different transition phases of the xerogel in accordance with the earlier conclusion that the metal ion penetrates into the alumina matrix and occupies aluminium sites. In boehmite and α-Al₂O₃ chromium is essentially in trivalent state, octahedrally coordinated, whereas in the transitionγ andδ phases tetrahedrally coordinated hexavalent state appears to be dominant. This agrees with the spinel structure expected for the transition phases.