Phase transformation of calcium phenyl phosphate in calcium hydroxyapatite

Calcium phenyl phosphate (CaPP) was synthesized from a mixture of Ca(OH)2 and phenyl phosphate (C6H5PO4H2) in an aqueous media. XRD pattern of CaPP exhibited five diffraction peaks at 2θ = 6.6, 13.3, 20.0, 26.8 and 33.7°. The d-spacing ratio of these peaks was ca. 1:1/2:1/3:1/4:1/5. The molar ratios of Ca/P and phenyl/P of CaPP were 1.0 and 0.92, respectively, and the chemical formula of the material was expressed as (C6H5PO4)0.92(HPO4)0.08Ca·1.3H2O, similar to that of dicalcium phosphate dihydrate (CaHPO4·2H2O: DCPD). These results allowed us to infer that CaPP is composed of a multilayer alternating bilayer of phenyl groups of the phosphates and DCPD-like phase. The structure of the material was essentially not altered after aging at pH 9.0–11.0 and 85 °C in an aqueous media. While, after aging at pH ≤8.0, the diffraction peaks of CaPP were suddenly weakened and disappeared at pH 7.0. Besides, new peaks due to calcium hydroxyapatite (Ca10(PO4)6(OH)2: Hap) appeared and their intensity was strengthened with decreasing the solution pH. TEM observation revealed that the Hap particles formed at pH 6.0 are fibrous with ca. 1.5 μm in length and ca. 0.2 μm in width. From these results, it is presumed that the layered CaPP was dissolved, hydrolyzed and reprecipitated to fibrous Hap particles at pH ≤8.0 and 85 °C in aqueous media. This phase transformation of CaPP in Hap resembled to the formation mechanism of Hap in animal organism.