Solid-state NMR spectroscopy of Pb-rich apatite

Pb-containing hydroxylapatite phases synthesized under aqueous conditions were investigated by X-ray diffraction and solidstate nuclear magnetic resonance (NMR) techniques to determine the Pb, Ca distribution. ³¹P and ¹H magic-angle spinning (MAS) NMR results indicate slight shifts of the isotropic chemical shift with increased Ca content and complex lineshapes at compositions with near equal amounts of Ca and Pb. ³¹P{ ²⁰⁷Pb} and ¹H{²⁰⁷Pb} rotational-echo double resonance (REDOR) results for intermediate compositions show that resolved spectral features cannot be assigned simply in terms of local Ca, Pb configurations or coexisting phases. ²⁰⁷Pb MAS NMR spectra are easily obtained for these materials and contain well-resolved resonances for crystallographically unique A1 and A2 Pb sites. Splitting of the A1 and A2 ²⁰⁷Pb resonances for pure hydroxylpyromorphite (Pb ₁₀(PO₄)₆(OH)₂) comparedtonatural pyromorphite (Pb₅(PO₄)₃Cl) suggests symmetry reduced fromhexagonal. We find that ²⁰⁷Pb{¹H} CP/MAS NMR is impractical in Pb-rich hydroxylapatites due to fast ²⁰⁷Pb relaxation.