Characterization of Variably Substituted Hydroxyapatites Using Low-Frequency Raman Spectroscopy

Low-frequency Raman spectroscopy was used to characterize variably substituted hydroxyapatites with Sr2+, F–, and CO32– as the substituting species in the crystalline and amorphous forms. These samples were characterized for substitution and crystallinity using methods previously explored in the literature (XRPD, IR, and Raman spectroscopy) to demonstrate a range of substitutions, and the levels of crystallinity were achieved. The sample series was characterized with low-frequency Raman spectroscopy where clear differences between amorphous and crystalline forms along with systematic changes in the crystalline spectra with the level of substitution were observed. Solid-state density functional theory was used to identify and characterize the observed low-frequency Raman modes, and principal component analysis further explored the trends in the data set. A new crystallinity index (CI) was proposed using the ratio of intensities at 140 and 110 cm–1 (CILFRintensityStokes) or −140 and −110 cm–1 (CILFRintensityanti-Stokes). When correlated to the XRPD-based CI, better correlation was observed with CILFRintensityStokes (NRMSE = 0.11, R2 = 0.85) and CILFRintensityanti-Stokes (NRMSE = 0.11, R2 = 0.88) compared to the previously used CIIR (NRMSE = 0.24, R2 = 0.03) or CIRaman (NRMSE = 0.15, R2 = 0.66) methods, which were particularly sensitive to the degree of substitution interfering with the level of crystallinity, thus making this a promising method for estimating crystallinity when dealing with variably substituted apatites.