Kinetics and mechanisms of the transformation of precipitated amorphous calcium phosphate with a Ca/P ratio of 1:1 to calcium pyrophosphates

The kinetics and mechanisms of crystallization, phase transformation and grain growth in amorphous calcium pyrophosphate (ACPP) derived from the fast nitrate synthesis were studied at 600-800 °C. The high-temperature calcium pyrophosphate α-CPP crystallized as the major and metastable phase (α/-CPP), and some stable β-CPP polymorph also formed. The activation energies were determined with 80 kJ/mol for α/-CPP grain growth, as well as with 525, 190 and 180 kJ/mol for β-CPP nucleation, formation and grain growth during α/-CPP to β-CPP transformation under isothermal annealing, respectively. The transformation processes were mainly proceeded by surface diffusion and additional volume diffusion as the temperature was increased. The metastable α/-CPP crystallization was associated with the Ostwald's step rule. The formation of two polymorphs from the same ACPP was assumed to occur from two different short-range order units formed in ACPP before the onset of crystallization.