Structure refinements of cadmium substituted lead fluoroapatites by powder X-ray pattern fitting

Solid solutions of cadmium and lead fluoroapatite [Pb(10−x)Cdx(PO4)6F2 (0⩽x⩽5)] were synthesized by a wet process in a basic medium. Replacement of lead by cadmium induces a linear variation of the crystallographic parameters “a” and “c” according to Végard's law. The cadmium content, as obtained from the refinement, is in agreement with the chemical analysis. The distribution of the lead and cadmium ions between two non-equivalent crystallographic sites, M(1) and M(2), was determined by the Rietveld method. The site-occupancy factors of atoms clearly indicate a preference of cadmium for site M(1) in the apatite structure in agreement with its smaller ionic radii. A progressive shift of the F− ion toward the center of the triangles formed by the site M(2) metals has been observed with increasing cadmium content.