Isomorphous Substitutions of Rare Earth Elements for Calcium in Synthetic Hydroxyapatites

Polycrystalline hydroxyapatites Ca10−xREEx(PO4)6(OH)2−xOx were synthesized and studied by X-ray powder diffraction, infrared absorption, diffuse-reflectance spectroscopy, and thermogravimetry. The solubility limits xmax of rare earth elements (REE) in Ca hydroxyapatites decreases with an increasing REE atomic number from xmax = 2.00 for La, Pr, and Nd to xmax = 0.20 for Yb at 1100 °C. Refinements of X-ray diffraction patterns by the Rietveld method show that REE atoms substitute for Ca preferentially at the Ca(2) sites of the apatite structure. The substitution decreases the Ca(2)−O(4) atomic distances in the calcium coordination polyhedra and increases the Ca(2)−O(1,2,3) distances. This observation shows that interatomic distances depend not only on radii of the ions involved in the substitution but also on their charges.