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 fluorapatite lattice). Smaller chemical shift perturbations observed by deconvolution of two shoulders at 61.2 and 58.8 ppm on the main 64.0 ppm resonance are assigned to a Ca3F configuration that has, respectively, either one or multiple Sr2+ neighbors in the adjacent Ca(2) sites. Quantitative peak intensity measurements relative to the main 64 ppm resonance of both the 79.6 ppm peak as well as the deconvoluted peaks separately indicate that Sr2+ ions preferentially occupy the Ca(2) site at a level 23% greater than that expected for random substitution in the above solid solution. A sample having the composition Ca4.95Sr5.05F2(PO4)6 has a qualitatively similar site preference and has peaks assigned to Ca2SrF at 70 ppm, CaSr2F at 87 ppm, and Sr3F at 105 ppm.