Phase Evolution during the Formation of α-Tricalcium Phosphate

The formation of α‐tricalcium phosphate from monobasic ammonium phosphate and calcium carbonate was investigated using a number of complementary techniques. Differential thermal analysis showed five distinct thermal events attributed to melting of the ammonium phosphate, decomposition of acidic calcium orthophosphate into an amorphous calcium metaphosphate, crystallization of β‐calcium metaphosphate, crystallization of β‐calcium pyrophosphate, and calcination of calcium carbonate. X‐ray diffraction analyses as a function of temperature supported the evolution of these events and phases and also showed the formation of other intermediates, including an amorphous phase, apatite, lime, and β‐tricalcium phosphate. Thermal gravimetric analysis (TGA) showed a large weight loss occurring between 150° and 250°C due to reaction between the acidic phosphate liquid and the calcium carbonate. This resulted in an amorphous intermediate with a Ca/P ratio between 0.5 and 1.0 from which both β‐calcium metaphosphate and β‐calcium pyrophosphate crystallized. Mass spectroscopy indicated that the ammonia and carbon dioxide were evolved in four different steps, while water was evolved in at least five steps. These decomposition steps correlated with those observed by TGA. Scanning electron microscopy indicated the formation of an intermediate phase that coated the calcium carbonate by 250°C. The proposed mechanistic reaction path to alpha‐tricalcium phosphate involves the formation and consumption of the following sequence of intermediates: an acidic amorphous condensed phosphate; β‐calcium metaphosphate and β‐calcium pyrophosphate; lime, apatite, and β‐tricalcium phosphate.