Synthesis and thermal decomposition of layered calcium phosphates including carboxylate ions

Octacalcium bis (hydrogenphosphate) tetrakis (orthophosphate) pentahydrate [Ca8(HPO4)2(PO4)4·5H2O] (OCP) consists of so-called “apatite” layers and “hydrate ” layers. We have researched on the synthesis of layered OCP including various kinds of carboxylate ions. Calcium hydrogen phosphate dihydrate [CaHPO4·2H2O] (DCPD) and α-tricalcium bis (orthophosphate) [Ca3(PO4)2] (α-TCP) were mixed with respective carboxylic acid such as succinic acid, glutamic acid, aspartic acid, and citric acid which are bioactive organic compounds. The mixture was hydrolyzed to OCP including carboxylate ions after reaction for 24 h at 40–60 °C. We found that the OCP including bioactive organic compound is easily formed under the presence of carboxylic acid without any control of pH. It was observed that the synthesized products consisted of small sized thin layers.Thermolysis of OCPs including carboxylate ions did not show significant change in the structure at lower temperature than 400 °C. At 600 °C, they decomposed into hydroxyapatite [Ca10(PO4)6(OH)2] (HAp) and β-TCP together with a small amount of β-dicalcium diphosphate [β-Ca2P2O7]. The respective amount of their reformed products depended on the content and the species of carboxylate ions incorporated into OCP.