HYDROTHERMAL TRANSFORMATION OF CALCIUM HYDROGEN PHOSPHATE DIHYDRATE INTO MAGNESIUM WHITLOCKITE

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We here report a single-step, rapid synthesis method for magnesium whitlockite (MgWH). The hydrothermal synthesis of MgWH by transformation of calcium hydrogen phosphate dihydrate (DCPD) in formic acid solution was investigated. The fraction of MgWH increased with increasing Mg²⁺ ion concentration. Single-phase MgWH with monodisperse, hexahedral-shaped particles was obtained when the reaction was conducted in the presence of 15 mmol/L Mg²⁺ ions and at 220 °C for more than 3 h. The results also indicated that the hydrothermal transformation into MgWH in the presence of Mg²⁺ ions was not pH-dependent precipitation but thermodynamically stable precipitation. Local structure analysis by ¹H and ³¹P nuclear magnetic resonance confirmed the presence of protonated phosphate (HPO₄^2-) and showed three distinct phosphate peaks corresponding to the MgWH structure, which were consistent with the crystal structure of conventional MgWH.