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 ³¹P, ¹⁹F, and ³⁵Cl solid-state magic angle spinning (MAS) NMR spectroscopic methods. ³¹P single-pulse (SP) NMR spectra show that for each composition the phosphate ³¹P 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 ¹⁹F SP NMR spectra reveal complex, broadened spectral profiles that are not correlated to composition. ¹⁹F{³⁵Cl} 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 ¹⁹F lineshapes are principally due to broad distributions of ¹⁹F-Ca distances arising from displacement of F along the anion channel. Approximate fluorine atomic positions are estimated based on ¹⁹F 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.