Determination of the Activation Energies of Phase Transition for Calcium Orthophosphates Based on Powder X-Ray Diffraction Data

Kinetic studies of the transformation of calcium orthophosphates metastable precipitates are performed under different synthesis conditions. The phase composition and degree of crystallinity are investigated by X-ray powder diffraction analysis. In acidic solution, precipitates of CaHPO (. )(4 )2H O-2 (DCPD) and CaHPO (4 )(DCPA) are formed at the early stage of precipitation, with the degree of crystallinity at the range of 17-35%. Specifically, DCPD precipitates at 30 degrees C and anhydrous DCPA at 50 degrees C. In alkaline solution (pH 8-10), only amorphous forms of calcium orthophosphate are precipitated, which is explained by the high degree of supersaturation. The effect of pH on the composition of precipitates is illustrated by the solubility isotherms of pure calcium orthophosphates. It is determined the activation energy of phase transformation of calcium orthophosphate from X-ray powder diffraction patterns. Based on this relationship developed, the activation energies for the recrystallization DCPD and DCPA are 10.2 and 13.1 kJ mol(-1), respectively, and for the phase transition of DCPD to DCPA - 36.7 kJ mol(-1). Further recrystallization to most thermodynamically stable hydroxyapatite occurs at the activation energy of 5.2 kJ mol(-1). These findings are critical to the phase transition and transformation of calcium phosphate minerals.