Hydroxyapatite Formation from Octacalcium Phosphate and Its Related Compounds: A Discussion of the Transformation Mechanism

Abstract

Abstract Octacalcium phosphate (OCP), which has a layered structure composed of an apatitic layer and a hydrated layer, is a precursor of hydroxyapatite (HAp). Although the topotactic transformation of OCP to HAp is a well-known phenomenon, its mechanism remains unclear. In this study, to clarify the role of the OCP hydrated layer in the mentioned transformation, we investigated the behavior of the transformation of OCP with hydrated layers of different thicknesses to HAp under hydrothermal conditions. We used three types of samples: plain OCP (Pure-OCP), OCP with incorporated succinate ions (Suc-OCP), and OCP with incorporated suberate ions (Sub-OCP). We found that all three OCP types transformed topotactically into HAp. The order of reactivity was Sub-OCP > Suc-OCP > Pure-OCP, which corresponds to the thickness of the hydrated layer. Since the transformation involves a compositional change, our finding implies that the hydrated layer acts as a diffusion (mass transfer) passage for dicarboxylate ions and hydrogen phosphate ions located within it, which move outside the crystal during the topotactic transformation. Our results thus contribute to a better understanding of the topotactic transformation mechanism of OCPs to HAp.