Isomorphous substitution of samarium and gadolinium for calcium in hydroxyapatite structure

Isomorphous substitution of samarium and gadolinium for calcium in synthetic hydroxyapatite by the scheme Ca2+ + OH− → Ln3+ + O2− was studied by X-ray powder diffraction and IR spectroscopy. The homogeneity region of Ca10 − xLnx(PO4)6(OH)2 − xOx phases of variable compositions synthesized at 1100°C was found to exist up to x of 1.8 for Sm and 1.4 for Gd. In the heterogeneous region of solid solutions, phases with Ln3PO7 and LnPO4 structures were formed. The structures of some solid solutions were refined by the Rietveld method, which allowed us to establish that the substitution of Ln3+ ions for Ca(2) positions is preferred. Changes in the intensities of the absorption bands of stretching and libration vibrations of OH groups are indicative of a partial dehydration of oxyhydroxyapatite. The substitution was shown to decrease Ca(2)-O(OH) and P-O distances and increase Ca(2)-O(1,2,3) distances.