Controls of Tricalcium Phosphate Single-Crystal Formation from Its Amorphous Precursor by Interfacial Energy

Different from the conventional solution precipitation, amorphous precursor involves widely in biomineralizations. It is believed that the development of crystalline structures with a well-defined shape in biological systems is essentially facilitated by the occurrence of these transient amorphous phases. However, the previous studies have not elucidated the physicochemical factors influencing the transformation from the transient phase into the stable phase. In this study, the evolutions from the amorphous calcium phosphate to the different-shaped (hexagon and octahedron; octahedron is an unexpected morphology of the crystal with space group of R3̅c) single crystals of β-tricalcium phosphate (β-TCP) were examined. The hexagonal β-TCP crystals were formed via the phase transformation of amorphous precursor in CaCl2−Na2HPO4-ethylene glycol solution; however, the octahedral β-TCP crystals were formed in Ca(OH)2-(NH4)2HPO4-ethylene glycol solution. Because the interfacial energies between amorphous phase and crystals were much smaller than those between solutions and crystals, the crystallization of the β-TCP phase occurred directly in the amorphous substrate rather than from the solution. It was interesting that the final morphology of product was also determined by the interfacial energy between the transformed crystal and solution. The current work demonstrated that the amorphous precursor epitaxial nucleation process and morphology selection of crystals in the amorphous phase could also be understood by an interfacial energy control. This result might provide an in-depth understanding of the biomimetic synthesis of crystals via a pathway of amorphous precursors.