X-ray powder diffraction, solid-state 31P-MAS-NMR and IR spectroscopy of cadmium-strontium mixed hydroxyapatites

Strontium-cadmium hydroxyapatite solid solutions CdxSr(10-x)(PO4)(6)(OH)(2) were prepared in aqueous medium. They were characterized by X-ray powder diffraction, P-31-MAS-NMR and IR spectroscopic analyses. The structural refinement, by X-ray powder pattern fitting (Rietveld method), indicates the limits of strontium-cadmium solid solutions (x less than or equal to 4) and allows us to determine the cadmium repartition in the apatite cell. Through the progressive replacement of Sr2+ (r = 1.32 Angstrom) by the smaller cation Cd2+ (r = 1.09 Angstrom), all interatomic distances decrease according to the decrease of the cell parameters. Hom ever, contrary to what could be expected from the coordinance of the metallic sites S-1 (hexacoordination) and S-2 (heptacoordination) the small cadmium cation does not preferentially occupy the four sites S-1: it is almost statistically distributed over both sites, with a slight preference for S-2. The P-31-solid-state-NMR of the pure phases Sr-10(PO4)(6)(OH)(2) and Cd-10(PO4)(6)(OH)(2) reveals a moderate chemical shift anisotropy (approximate to 20 ppm) corresponding to slight distortion of the phosphate tetrahedron. This observation holds also for the mixed species where the surrounding of the phosphate anion is expected to change significantly when substituting Cd for Sr. Through the progressive replacement of strontium by cadmium, the isotropic signal moves regularly to high frequency, according to the deshielding observed for Cd-10(PO4)(6)(OH)(2)(delta(iso) = 12.2 ppm) with respect to Sr-10(PO4)(6)(OH)(2) (delta(iso) = 2.9 ppm). Moreover the isotropic signal broadens considerably because of unresolved chemical shift distributions in the highly disordered material. (C) 2000 Academie des sciences / Editions scientifiques et medicales Elsevier SAS.