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

Abstract

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 int...