Dissolution-precipitation synthesis and thermal stability of magnesium whitlockite

Magnesium whitlockite (Mg-WH, Ca18Mg2(HPO4)(2)(PO4)(12)) is a promising candidate for biomedical application in bone regeneration; however, the fabrication of Mg-WH bioceramics by conventional methods is limited. Mg-WH is known to be thermally unstable and decomposes upon heating. The mechanism of thermal decomposition and phase evolution has not comprehensively been investigated so far. In the present work, Mg-WH was synthesized by a dissolution-precipitation process under hydrothermal conditions. Thermally induced degradation of the synthesized powders was investigated in detail by combining X-ray diffraction (XRD) analysis, infrared spectroscopy (FTIR), Raman spectroscopy as well as H-1 and P-31 solid-state nuclear magnetic resonance (NMR). The as-prepared Mg-WH powders were annealed at different temperatures in the range from 400 to 1300 & DEG;C. It was found that thermal decomposition starts at around 700 & DEG;C with the formation of beta-tricalcium phosphate (& beta;-TCP, Ca-3(PO4)(2)) and a mixture of two Ca2P2O7 polymorphs. Thermal decomposition occurs gradually and the co-existence of both Mg-WH and Mg-substituted & beta;-TCP phases was observed in a wide temperature range up to 1200 & DEG;C. Complete disappearance of the HPO42- structural unit was confirmed only after annealing at 1300 & DEG;C followed by melting at 1400 & DEG;C.