Synthesis, X‐ray Structural Analysis and Spectroscopic Investigations (IR and 31P MAS NMR) of Mixed Barium/Strontium Fluoroapatites

Continuous solid solutions of barium/strontium fluoroapatites Ba(10−x)Srx(PO4)6F2 (Ba(10−x)SrxFAp; 0 ⩽ x ⩽ 10) were obtained in an aqueous medium. They were investigated by Rietveld structure-refinement methods using powder X-ray diffraction patterns and by chemical analysis, IR spectroscopy and 31P MAS NMR spectroscopy. Replacement of barium by strontium induces a regular decrease of the lattice constants a and c according to Vegard’s law. The results of the powder-fitting structure refinements indicate a clear preference of strontium for the S(1) site of the apatitic structure. A high-wavenumber shift of the IR absorption bands characteristic of PO4 internal modes is also observed, in agreement with the diminution of the ionic character of the M−O bonds. For increasing strontium content, a regular δiso increase is shown in the 31P MAS NMR spectra, from δ = 0.91 ppm for Ba10FAp to δ = 3.06 ppm for Sr10FAp. For all mixed Ba/Sr fluoroapatites, the low intensities of the spinning bands reflect the weak 31P chemical-shift anisotropy, in agreement with the low distortion of the PO4 tetrahedron, as shown by Rietveld refinement. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)