Structural Transformations in Mechanochemically-Synthesized Strontium-Doped Hydroxyapatite

This study aims to investigate crystallization of strontium-substituted hydroxyapatite upon sintering. A study of mechanochemically-synthesized hydroxyapatite (HAp), doped with strontium at varying levels from 0 to 20 % is presented. The Sr-doped HAp powders were obtained by a mechanochemical method, followed by sintering at 1000 °C. The sintered samples were characterized using a complimentary set of SEM-EDX, XRD, FTIR, Raman, and solid-state NMR techniques. The FTIR spectra of the sintered samples showed a decreasing ν OH vibrational bands with increasing Sr content. This conflict is resolved by multinuclear solid-state NMR which conclusively demonstrates presence of OH⁻ in all samples, and that the strontium doping does not influence the OH⁻ concentration. Sr doping is demonstrated to not be random, with evidence of clustering, but without phase segregation even at the higher Sr loadings. The structural transformations of HAp revealed herein have significant implications in ongoing development of hydroxyapatite based systems, in monitoring the degree of hydroxylation, phase structure and concomitant effects on crystallinity and bioactivity.