Crystallographic study of lead-substituted hydroxyapatite synthesized by high-temperature mixing method under hydrothermal conditions

The solid solutions in the system of Ca and Pb hydroxyapatite, Ca10−χPbχHA (χ = 0–10), were successfully synthesized by high-temperature mixing method (HTMM) at 200 °C for 12 h under hydrothermal conditions. The samples were characterized by X-ray diffraction, chemical analysis and electron microscopic observation, and the site of the metal ions in the solid solutions was analyzed with the Rietveld method. The lattice parameters of the solid solutions prepared by both of the methods didn’t vary linearly with Pb contents, and they have two turning points of 0.4 and 0.6 of Pb content. It was found that Pb ions in the solid solutions preferentially occupied the M (2) site in the apatite structure. HTMM gives Ca–Pb HA solid solutions much better crystallization. However, due to the formation of intermediate compound of Pb3O2(OH)2 in the Pb(NO3)2·4H2O solution before mixing with (NH4)2HPO4 solution at 200 °C, HTMM causes the decrease of crystallization of the samples with high Pb content.