R-line luminescence from trace amounts of Cr3+ in aluminum oxide and its dependence on sample hydration

The R-line luminescence is observed in the Raman spectra of alpha-and gamma-Al2O3 with chromium concentrations of less than 1 ppm. The same signals were previously used to monitor the setting reaction of dental cements. With laser excitation <690 nm, the R-line luminescence can be observed in the optical spectra of most commercial products containing alpha-Al2O3. It is found that the intensities of R-line signals are related to the degree of hydration of the alpha-Al2O3 sample, which can be explained by the formation of additional octahedral sites for Cr3+ near the sample surface involving oxygen atoms from the Al2O3 matrix and from absorbed water molecules.