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

Abstract

Kinetic studies of the transformation of calcium orthophosphates metastable precipitates were performed under different synthesis conditions. Phase composition and degree of crystallinity were investigated by X-ray powder diffraction analysis. In acidic solution, precipitates of CaHPO42H2O (DCPD) and CaHPO4 (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C and anhydrous DCPA at 50°C. In alkaline solution (pH 8-10), only amorphous forms of calcium orthophosphate is precipitated, which is explained by the high degree of supersaturation (i.e., the high rate of precipitation compared to that in acidic media). The diffraction peaks of DCPD and DCPA are found to be 0.3-0.45 degrees lower relative to their reference data, which is caused by decrease of lattice strain during early stage of crystallization. Furthermore, the initial molar ratio of Ca/P in reagent mixture was found to play subordinary role in determining the composition of final calcium phosphate precipitates.The effect of pH on the composition of precipitates is illustrated by the solubility isotherms of pure calcium orthophosphates. Given that the intensities of diffraction peaks are proportional to planar density of the material in the given plane, we propose, for the first time, to determine activation energy of phase transformation of calcium orthophosphate from X-ray powder diffraction patterns. Based on this relationship developed, the activation energy for the recrystallization DCPD and DCPA are 10.2 and 13.1 kJ/mol, respectively and for the phase transition of DCPD to DCPA -36.7 kJ/mol. Further recrystallization to most thermodynamically stable Ca10(PO4)6(OН)2 hydroxyapatite (HA) occurs at the activation energy of 5.2 kJ/mol. These findings are critical on phase transition and transformation of calcium phosphate minerals.

Citrate-Stabilized Amorphous Calcium Phosphate Nanoparticles Doped with Micronutrients as a Highly Efficient Nanofertilizer for Environmental Sustainability

“…Citrate-free ACP with Ca/P ratio ∼1.67 undergoes a phase transition to less soluble HA over time or at elevated temperatures . To determine the temperature and mechanism of this transition for the citrate-stabilized ACPc, we monitored the changes in the IR spectra and weight loss during thermal treatments (Figure ).…”

3D‐printed degradable hydroxyapatite bioactive ceramics for skull regeneration

“…When it comes to thermodynamic stability, HA is the calcium phosphate crystalline phase with the greatest longevity. Besides other calcium phosphate components, HA most closely resembles the inorganic component of human bone tissues 28 . To stimulate bone repair, implanted materials may lead mesenchymal stem cells and osteoblast precursors from the surrounding connective tissue to differentiate into osteoblasts 29 .…”

“…Besides other calcium phosphate components, HA most closely resembles the inorganic component of human bone tissues. 28 To stimulate bone repair, implanted materials may lead mesenchymal stem cells and osteoblast precursors from the surrounding connective tissue to differentiate into osteoblasts. 29 Bone-inducing materials implanted into bone tissue may hasten bone healing and reduce repair time.…”

Role of Cation Substitution and Synthesis Condition in a Calcium Phosphate-Based Novel Nanofertilizer on Lettuce (Lactuca sativa) Yield

“…This is because an additional H + ion in the structure facilitates the dissolution by reducing the isoelectric point (IEP) from 8.2 to a lower pH value. At pH values above the IEP, the HA is thermodynamically stable and tends to precipitate in saturated solution independently of the Ca/P ratio or the type of precursors involved . Conversely, when the pH is below the IEP, depending on the nature of precursors, other Ca–P products, such as dicalcium phosphate CaHPO 4 (DCPA), can form.…”

“…At pH values above the IEP, the HA is thermodynamically stable and tends to precipitate in saturated solution independently of the Ca/P ratio or the type of precursors involved. 18 Conversely, when the pH is below the IEP, depending on the nature of precursors, other Ca−P products, such as dicalcium phosphate CaHPO 4 (DCPA), can form. The solubility of DCPA is >10 times higher than HA because it possesses one H + per Ca 2+ cation in the chemical formula, although the solubility is still approximately 100 times lower than that of superphosphate.…”