Étude par diffraction des rayons X, par résonance magnétique nucléaire 31P en phase solide et par spectrométrie infrarouge des hydroxyapatites mixtes cadmium–strontium

Abstract

Abstract X-ray powder diffraction, solid-state 31 P-MAS-NMR and IR spectroscopy of cadmium–strontium mixed hydroxyapatites. Strontium–cadmium hydroxyapatite solid solutions Cd x Sr (10– x ) (PO 4 ) 6 (OH) 2 were prepared in aqueous medium. They were characterized by X-ray powder diffraction, 31 P-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 ≤4) and allows us to determine the cadmium repartition in the apatite cell. Through the progressive replacement of Sr 2+ ( r =1.32 A) by the smaller cation Cd 2+ ( r =1.09 A), all interatomic distances decrease according to the decrease of the cell parameters. However, 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 31 P- solid-state-NMR of the pure phases Sr 10 (PO 4 ) 6 (OH) 2 and Cd 10 (PO 4 ) 6 (OH) 2 reveals a moderate chemical shift anisotropy (≈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 (PO 4 ) 6 (OH) 2 ( δ iso =12.2 ppm) with respect to Sr 10 (PO 4 ) 6 (OH) 2 ( δ iso =2.9 ppm). Moreover the isotropic signal broadens considerably because of unresolved chemical shift distributions in the highly disordered material. © 2000 Academie des sciences / Editions scientifiques et medicales Elsevier SAS hydroxyapatite / Rietveld refinement / 31 P-MAS-NMR / cadmium / strontium / phosphate