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 ¹H and ³¹P solid-state nuclear magnetic resonance (NMR). The as-prepared Mg-WH powders were annealed at different temperatures in the range from 400 to 1300 °C. It was found that thermal decomposition starts at around 700 °C with the formation of beta-tricalcium phosphate (β-TCP, Ca₃(PO₄)₂) and a mixture of two Ca₂P₂O₇ polymorphs. Thermal decomposition occurs gradually and the co-existence of both Mg-WH and Mg-substituted β-TCP phases was observed in a wide temperature range up to 1200 °C. Complete disappearance of the HPO₄²⁻ structural unit was confirmed only after annealing at 1300 °C followed by melting at 1400 °C.