Counterions present in syntheses induce the precipitation of two different populations of Sr-containing hydroxyapatite crystals

The ions found in biological environment during biomineralization as well as the counterions present in syntheses, i.e., the reagents employed, play an important role in the precipitation and stabilization of apatites. In this work, precipitation of strontium-containing hydroxyapatites at different concentrations (0, 20, 40, 60, 80 and 100% Sr²⁺) was performed in the presence of two counterions typically present in biological environments: Na⁺ and Cl⁻. It was demonstrated that the presence of Na⁺/Cl⁻ in the synthesis environment led to the precipitation of a biphasic hydroxyapatite (HAp) system formed by two non-miscible phases: Ca-rich HAp and Sr-rich HAp. The biphasic system was observed for intermediate Sr²⁺ concentrations (20, 40 and 60%) and exhibited greater lattice microstrain compared with the single-phase systems of Ca HAp (0%) and Sr HAp (100%). Although Na⁺/Cl⁻ were inserted into both Ca- and Sr-rich HAp phases, Cl⁻ ions were preferentially accommodated into the enlarged Sr-rich HAp structure. The presence of Cl⁻ ions in the Sr-rich HAp phase decreased microstrain compared to the Ca-rich phase. After calcination, the biphasic system was transformed into a completely miscible Ca/Sr HAp phase without the formation of other phosphates or oxides.