Characterization of Variably Substituted Hydroxyapatites Using Low-Frequency Raman Spectroscopy

Low-frequency Raman spectroscopywas used to characterizevariablysubstituted hydroxyapatites with Sr2+, F-, and CO3 (2-) as the substituting speciesin the crystalline and amorphous forms. These samples were characterizedfor substitution and crystallinity using methods previously exploredin the literature (XRPD, IR, and Raman spectroscopy) to demonstratea range of substitutions, and the levels of crystallinity were achieved.The sample series was characterized with low-frequency Raman spectroscopywhere clear differences between amorphous and crystalline forms alongwith systematic changes in the crystalline spectra with the levelof substitution were observed. Solid-state density functional theorywas used to identify and characterize the observed low-frequency Ramanmodes, and principal component analysis further explored the trendsin the data set. A new crystallinity index (CI) was proposed usingthe ratio of intensities at 140 and 110 cm(-1) (CILFRintensityStokes) or -140 and -110 cm(-1) (CILFRintensityanti-Stokes). When correlated tothe XRPD-based CI, better correlation was observed with CILFRintensityStokes (NRMSE = 0.11, R (2) = 0.85) and CILFRintensityanti-Stokes (NRMSE = 0.11, R (2) = 0.88) compared to the previously used CIIR (NRMSE = 0.24, R (2) = 0.03) or CIRaman (NRMSE = 0.15, R (2) = 0.66)methods, which were particularly sensitive to the degree of substitutioninterfering with the level of crystallinity, thus making this a promisingmethod for estimating crystallinity when dealing with variably substitutedapatites. Low-frequency Raman spectroscopy andsolid-state densityfunctional theory were used to characterize variably substituted hydroxyapatiteswith Sr2+, F-, and CO3 (2-) as the substituting species in the crystalline andamorphous forms.