Amorphous calcium phosphate often forms as a precursor phase in a solution at sufficiently high supersaturation and pH, and then transforms to the thermodynamically stable hydroxyapatite. The chemical composition, structure, and property of the amorphous phase are dependent on the structure of its composing clusters. Based on the results from the measurements of in situ Ca K-edge X-ray near-edge structure and ex situ X-ray diffraction, as well as the concomitant pH change in the reaction process, here we propose an improved model for the structure of “Posner’s cluster” and identify the three types of reactions that lead to the formation of amorphous calcium phosphate and its subsequent transition to crystalline hydroxyapatite.

Calcium phosphate crystallization is of importance in medicine and materials science, and aggregation frequently happens at its early stage. Herein, we report a kinetic study of crystallization in particles of aggregated nanocrystals of calcium phosphate by in situ synchrotron small-angle X-ray scattering. The particles were random fractal aggregates, having diameters of 20–60 nm and comprising structural units of 2.5 nm in dimension. With the application of a logistic model, the structural changes of the particles were characterized by the successive developments of mass and surface fractal dimensions. The rate constant of mass fractal development was found to be 0.052 h–1, corresponding to the generation of crystallites on the pre-existing seeds inside an aggregate. These newly formed nanocrystals then detached from their seeds and fell into the voids. Afterward, the surface fractal structure developed at a rate of 0.11 h–1, making the particle surface less rough. During this period, a transition layer of 1.2-nm thickness emerged between a particle and the surrounding solution. Our findings present an advance toward understanding the crystallization in particles of aggregated nanocrystals and would be beneficial for preparing nanocrystals and recognizing the roles of other substances (such as proteins and metals) in both biological and synthetic systems.