Complex F,Cl Apatite Solid Solution Investigated Using Multinuclear Solid-State NMR Methods

A series of synthetic F,Cl apatites with low hydroxyl content was investigated using P-31, F-19, and Cl-35 solid-state magic angle spinning (MAS) NMR spectroscopic methods. P-31 single-pulse (SP) NMR spectra show that for each composition the phosphate P-31 chemical shift depends on occupancy of the nearest anion site for each composition. Overall the average chemical shift deviates from linear across the F/Cl series. For intermediate compositions F-19 SP NMR spectra reveal complex, broadened spectral profiles that are not correlated to composition. F-19{Cl-35} TRAPDOR results indicate that the spectral profiles do not reflect occupancy of F and Cl in adjacent column anion sites. We propose that the complex F-19 lineshapes are principally due to broad distributions of F-19 Ca distances arising from displacement of F along the anion channel. Approximate fluorine atomic positions are estimated based on F-19 chemical shifts of alkaline earth fluoride salts, and the results are in good accord with those proposed for intermediate composition F,Cl apatites from XRD data.