Structure of vanadate in calcium phosphate and vanadate apatite solid solutions

Abstract

Polycrystalline solid solutions of phosphate and vanadate calcium apatites were synthesized and studied by XRD, XPS, ³¹P and ⁵¹V NMR, FTIR and UV spectroscopies. Homogeneous distributions of vanadium within the solid and the surface were obtained. While ⁵¹V NMR spectra suggest an axial symmetry of the vanadate group, FTIR spectra indicate distortion from essentially C_3v symmetry in solid solutions with low vanadium content to C_s for pure vanadate apatite, in agreement with the site symmetry group of phosphate in hydroxyapatite. Refinements of XRD pattern of vanadate apatite by Rietveld method confirm the C_s point group attribution. Three vanadium oxygen bond lengths were found around 168 pm and one close to 174 pm, suggesting that the C_s point group could be generated by a small distortion from C_3v symmetry. Analysis of UV spectra confirms that distortion from T_d symmetry increases with vanadium content and suggests some contraction of the vanadate ion in the apatite lattice.