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 α- and γ-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 α-Al₂O₃. It is found that the intensities of R-line signals are related to the degree of hydration of the α-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.