The influence of annealing conditions on the Ca/P ratio and phase transformations in bulk calcium phosphates

The present study investigates the influence of annealing conditions on phase transformations in calcium phosphates. Synthetic brushite (CaHPO₄·2H₂O) with a Ca/P ratio of 1 : 1 was taken as a starting material and annealed at 1200 °C for different periods of time under three different conditions: in a static air atmosphere, in a nitrogen flow and in a vacuum. It was demonstrated that the phase composition and Ca/P ratio in the final product highly depend on the annealing conditions due to the evaporation of phosphate species. Annealing in air did not result in significant changes in the Ca/P ratio, and α-Ca₂P₂O₇ was obtained as a major phase with a minor α-Ca₃(PO₄)₂ phase. A similar phase composition was obtained after the heat treatment in a nitrogen flow. The most drastic changes in stoichiometry and phase composition were obtained after annealing in a vacuum. The resulting powders consisted mostly of α-Ca₃(PO₄)₂ with a negligible amount of α-Ca₂P₂O₇ as a secondary phase, which suggests a remarkable change in the Ca/P ratio up to approximately 1.5 : 1. The evaporation of phosphate species from the bulk powders occurred exclusively from α-Ca₂P₂O₇, and no obvious degradation of α-Ca₃(PO₄)₂ was observed under the selected conditions.