19F MAS NMR Investigation of Strontium Substitution Sites in Ca2+/Sr2+ Fluorapatite Solid Solutions

The partial replacement of Ca2+ by Sr2+ in the fluorapatite lattice results in additional peaks in the 19F MAS NMR spectra at 9.4 T other than the main resonance of Ca10F2(PO4)6 at 64.0 ppm (from hexafluorobenzene). The assignment of these peaks to specific structural configurations is possible in the sample containing the least strontium, with a composition of Ca8.97Sr1.03F2(PO4)6. The solid-solution character of this sample is established by the observation of spectral spin diffusion between various peaks in the SPARTAN (selective population anti-z and rate of transfer to adjacent nuclei) experiment. Calculations based upon modeling the 19F chemical shift tensor show that this process is facilitated for most crystallites by the close approach of two peaks' resonances during the rotor cycle. A peak and set of spinning sidebands with an isotropic chemical shift of 79.6 ppm is assigned to fluoride ions in the center of a triangle of Ca2Sr ions (the so-called Ca(2) sites occurring as Ca3F in the fluorapatit...