Hydrogen Environments in Low-OH, F, Cl Apatites Revealed by Double Resonance Solid-State NMR

A series of hexagonal low-OH, F, Cl apatite compositions synthesized by high-temperature reaction is characterized by powder X-ray diffraction (PXRD) and ¹H magic-angle spinning (MAS) solid-state NMR methods. Quantitative ¹H NMR analysis indicates that the hydroxyl content in all samples is low, less than 2 mol percent, and is due to OH in apatite. ¹H{³¹P} rotational echo double resonance (REDOR) NMR difference spectroscopy indicates that hydroxyl groups are preferentially oriented toward fluorine, probably because of OH^-···F^- hydrogen bonding. Clustered hydroxyl groups are also observed and may play a significant role in hexagonal solid solution for ternary F, OH, Cl apatites.