Influence of thermal treatment on the structure of calcium hydroxyapatite

Abstract

Synthetic calcium hydroxyapatite, Ca10(PO4)6(OH)2 (CaHap), prepared by a wet method was evacuated at various temperatures up to 900°C and the influence of thermal treatment on the structure of this material was investigated by FTIR, determination of concentration of surface P–OH groups, adsorption of H2O and heat of immersion in water. The number of surface P–OH groups was 2.6 groups nm−2 and the ionic ratio of surface HPO42− to H2PO4− was 31:69. The surface P–OH groups were essentially dehydroxylated by outgassing above 400°C to produce surface P–O–P species. The heat of rehydroxylation per one P–O–P group was determined to be 1.43×10−16 J group−1. When outgassed above 600°C, CaHap was rehydrated via the reaction: (1−x)Ca10(PO4)6(OH)2+3xβ-Ca3(PO4)2+xH2O→Ca10−x(P2O7)x(PO4)6−2x(OH)2.