Crystal-Chemical Characteristics of Silicon−Neodymium Substituted Hydroxyapatites Studied by Combined X-ray and Neutron Powder Diffraction

Neodymium−silicon britholites, Ca 10 - x Nd x (PO 4 ) 6 - x (SiO 4 ) x (OH) 2, have been synthesized by the ceramic method. These materials can be used as host matrixes for actinides immobilization and Nd models the actinide. The crystal-chemistry evolution as a function of the Si and Nd content has been studied by combined neutron and X-ray Rietveld refinement. The SiO 4 4 - substitution for PO 4 3 - does not lead to significant changes in the PO 4 tetrahedrons. The Nd 3+ cations substitute for Ca 2+ and are placed at the Ca(2) sites, getting closer to the hydroxyl sites. The local increase of positive charge around hydroxyls leads to the O 2 - stabilization in 4e Wyckoff position. Since neutrons are sensitive to the H atoms, the hydroxylation degree has been calculated. Finally, the scattering density Fourier difference maps suggest the presence of HPO 4 2 - groups, which partially substitute for PO 4 3 - to keep the electric balance of these compounds.