Preferential occupancy of metal ions in the hydroxyapatite solid solutions synthesized by hydrothermal method

The solid solutions in the systems of Ca-Cd HAp [Ca10−xCdxHAp (x = 0–10)], Ca-Sr HAp [Ca10−xSrxHAp (x = 0–10)] and Ca-Pb HAp [Ca10−xPbxHAp (x = 0–10)], were successfully synthesized at 200 °C for 12 h under hydrothermal conditions. The site of the metal ions in the solid solutions was analyzed by the Rietveld method. The results of the Rietveld analysis indicated that the metal ions of Pb2+, Sr2+, and Cd2+ all preferentially occupied M (2) sites in the apatite structure. The preferential occupancy of the metal ions in M (2) sites were explained mainly by their ionic radius and electronegativity.