Luminescence of Cr³⁺ in natural and calcined diaspore

The luminescence of Cr³⁺ in natural and calcined diaspore is applied to probe structural properties of the phases. Steady-state luminescence spectra of the natural diaspore [α-AlO(OH)] are typical of ²E→⁴A₂ transitions of Cr³⁺ luminescence centers (R-lines) substituting for Al³⁺ in six-fold coordination. The dehydroxylated phase of the diaspore is characterized by a low degree of crystallinity. This phase is not detectable by XRD and Raman spectroscopy up to 900°C. The time-resolved luminescence spectra of the diaspore treated at 900°C showed two variations: the first is an R-line of Cr³⁺ luminescence in octahedral sites with a strong crystal field in a very disordered environment, and the second is a broad band of Cr³⁺ luminescence in weaker crystal field sites. The crystalline phase of corundum [α-Al₂O₃] is detected by XRD and Raman spectroscopy after treating the diaspore at 1000°C and above (up to 1300°C). The lines in the spectrum of the diaspore treated at this temperature are typical of Cr³⁺ luminescence centers (R-lines) in Cr-corundum (ruby) structure.