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→Ca_10−x(P₂O₇)_x(PO₄)_6−2x(OH)₂.