Local structure of low-temperature γ-Al2O3 phases as determined by the luminescence of Cr3+ and Fe3+

The γ and γ* phases of alumina synthesized from crystalline boehmite and pseudoboehmite were shown to strongly differ in some physicochemical characteristics. Photoluminescence of Cr3+ and Fe3+ as luminescent probes with their natural impurity concentrations was detected in the tested alumina samples and the local structure of the samples was examined. The crystal field strength and Racah parameters in γ-Al2O3 phases were determined. The ratio of octahedral and tetrahedral cationic positions was equal to 1 for γ-Al2O3 synthesized from crystalline boehmite, and 0.3 for that obtained from pseudoboehmite. The effect of residual OH groups and weakly bound molecular water on the local structure of γ-Al2O3 phases was revealed. The luminescence of Cr3+ was detected in subsurface structures of each alumina; defects produced by nanocrystallites were shown to affect the luminescence.