Peculiarities in thermal evolution of precipitated amorphous calcium phosphates with an initial Ca/P ratio of 1:1

Thermal evolution of amorphous calcium phosphate (ACP) powder from a fast nitrate synthesis with a Ca/P ratio of 1: 1 were studied in the range of 20-980 degrees C. The powder consisted of amorphous dicalcium phosphate anhydrate (CaHPO4) after heating to 200 degrees C. CaHPO4 gradually condensed to amorphous calcium pyrophosphate Ca2P2O7 (CPP) between 200 to 620 degrees C. Amorphous CPP crystallized at 620-740 degrees C to a metastable polymorph alpha'-CPP of the high-temperature phase alpha-CPP and beta-CPP. The alpha'-CPP/beta-CPP phase ratio reached a maximum at 800 degrees C (60 wt% alpha'-CPP/40 wt% beta-CPP), and alpha'-CPP gradually transformed to beta-CPP at a higher temperature. Some beta-TCP occurred at 900 degrees C, so that a three-phasic mixture was obtained in the powder heated to 980 degrees C. The occurrence of metastable alpha'-CPP is attributed to Ostwald's step rule, and a mechanism for beta-TCP formation is proposed. The advantages of prospective biomaterials from these powders are discussed.