Competition and Neutralization: Thermally Induced Crystallization and Phase Evolution of Amorphous Calcium Phosphate with Cosubstitution of Larger and Smaller Divalent Cations

The presented study evaluated the effect of cosubstitution of larger and smaller divalent cations on the thermally induced crystallization of amorphous calcium phosphate (ACP). The predesigned combinations of larger (Sr²⁺ and Ba²⁺) and smaller (Mg²⁺, Cu²⁺, and Co²⁺) divalent cations were carried out and their effects on the thermodynamic equilibrium between α/β-tricalcium phosphate (TCP) were outlined. The coexistence of larger and smaller divalent cations shielded the formation of α-TCP and shifted the thermodynamic equilibrium toward the β-TCP, which implied that the smaller cations dominated the crystalline phase. However, the retarded crystallization induced by the larger cations still remained and allowed ACP to maintain its amorphous nature partly or completely until a higher temperature.