Abstract
The phase transformation of γ‐Al~2~O~3~ at high temperatures was characterized using X‐ray diffraction (XRD) and fluorescence techniques. The results of a comparative fluorescence spectroscopic study of commercial and pure Al~2~O~3~ show that with laser excitation at 632.8 and 514.5 nm the absorption bands at 683.8 and 686.7 nm (R‐line luminescence) are attributed to the presence of Cr^3+^ ions in octahedral coordination in a θ‐Al~2~O~3~ environment. The other two bands at 692.8 and 694.4 nm (R~2~ and R~1~ lines) are attributed to Cr^3+^ fluorescence in an α‐Al~2~O~3~ environment. Compared with XRD measurements, fluorescence spectroscopy has a higher sensitivity for identification of phase transformations in θ‐Al~2~O~3~ and α‐Al~2~O~3~. According to the different absorptions of Al~2~O~3~ with two laser excitations, the laser excitation at 514.5 nm is found to be more sensitive at the surface region while the laser excitation at 632.8 nm supplies information about the bulk. The fluorescence spectra together with UV–visible diffuse reflectance spectra and XRD patterns indicate that the γ‐Al~2~O~3~ starts transforming to θ‐Al~2~O~3~ and α‐Al~2~O~3~ simultaneously at 800 °C. However, the θ‐Al~2~O~3~ and α‐Al~2~O~3~ compositions of the surface region and the bulk are different. The α‐Al~2~O~3~ content at the surface region is higher than that in the bulk. The γ‐Al~2~O~3~ sample transforms to α‐Al~2~O~3~ completely at 1250 °C. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)