Crystallinity and compositional changes in carbonated apatites: Evidence from ³¹P solid-state NMR, Raman, and AFM analysis

Solid-state (magic-angle spinning) NMR spectroscopy is a useful tool for obtaining structural information on bone organic and mineral components and synthetic model minerals at the atomic-level. Raman and ³¹P NMR spectral parameters were investigated in a series of synthetic B-type carbonated apatites (CAps). Inverse ³¹P NMR linewidth and inverse Raman PO ₄ ^3- ν₁ bandwidth were both correlated with powder XRD c-axis crystallinity over the 0.3-10.3 wt% CO₃ ^2- range investigated. Comparison with bone powder crystallinities showed agreement with values predicted by NMR and Raman calibration curves. Carbonate content was divided into two domains by the ³¹P NMR chemical shift frequency and the Raman phosphate ν₁ band position. These parameters remain stable except for an abrupt transition at 6.5 wt% carbonate, a composition which corresponds to an average of one carbonate per unit cell. This near-binary distribution of spectroscopic properties was also found in AFM-measured particle sizes and Ca/P molar ratios by elemental analysis. We propose that this transition differentiates between two charge-balancing ion-loss mechanisms as measured by Ca/P ratios. These results define a criterion for spectroscopic characterization of B-type carbonate substitution in apatitic minerals.