Structural Characterization of Iodate and Carbonate Substituted Calcium Hydroxyapatites: Insights From XRD, SEM, and TEM

This study investigates the structure and microstructure of hydroxyapatite substituted with iodate (IO 3 − ) and/or carbonate (CO 3 2− ) ions, within the context of long‐lived radionuclide waste conditioning. The role of the synthesis protocol, “reverse” or “direct” precipitation from aqueous solution, as well as the nature and amount of substituted anions on the structure and microstructure of hydroxyapatite samples, was evaluated using XRD, SEM, and HRTEM. The anisotropic crystallite shape assessed by HRTEM analyses was taken into account for the Rietveld refinements of the XRD patterns. Considering the reverse synthesis protocol, these refinements revealed the formation of a single hydroxyapatite phase. Despite the use of long counting times, no crystalline secondary phases were detected by XRD, regardless of the nature and amount of the substituting anions. Iodate ions substituting for A sites tend to expand the lattice, while carbonate ions located in B sites induce lattice contraction. Clearly, the influence of iodate substitution on the structural modifications is lower than that of carbonate substitution. Moreover, co‐substitution seems to modify the incorporation mode of iodate ions within the structure. The intended application of this work requires studying high substitution rates, thereby addressing gaps in the literature regarding the structural evolution of hydroxyapatite with increasing substitution rates. In this study, substitution levels reached up to 15.4 wt.% for iodate mono‐substitutions, 11.9 wt.% for carbonate mono‐substitutions, and approximately 20 wt.% for combined iodate and carbonate ions co‐substitutions.