Phase Formation and Stability of Whitlockite in the Ca（OH）2-Mg（OH）2-H3PO4 Ternary System via LowTemperature Wet Precipitatio

Abstract

Calcium and magnesium orthophosphates have been widely studied in biomedical and industrial fields due to their excellent biocompatibility and high specific surface area. In particular, Whitlockite （WH, Ca18Mg2（HPO4）2（PO4）12）, which is rarely found on Earth, is an important phase in the Ca–Mg phosphate system. It has various intermediate phases, making its formation conditions highly sensitive. Therefore, in this study, phosphate phases formed in the Ca（OH）2–Mg（OH）2–H3PO4 ternary system were investigated via a low-temperature wet precipitation method, focusing on the conditions for forming single-phase WH. The Ca:Mg molar ratio, （Ca + Mg）/P ratio, and pH were selected as key variables, and the resulting phases were analyzed. The synthesized powders were characterized using XRD, Raman, FTIR, SEM-EDS, and ICP-OES. The results confirmed the formation of single-phase WH in the range of Ca:Mg = 1:1–3.17:1, with the highest crystallinity obtained at Ca:Mg = 2.85:1. Within the compositional range where single-phase WH was formed, an increase in Ca content favored the formation of Hydroxyapatite （HAp, Ca10（PO4）6（OH）2） or Dicalcium phosphate anhydrous （DCPA, CaHPO4）, while an increase in Mg content led to magnesium phosphates such as Newberyite （NB, MgHPO4·3H2O）. Furthermore, secondary phases such as HAp or DCPA were also observed when the （Ca + Mg）/P ratio was not 1.0. In addition, at pH values above 5.5, calcium or magnesium phosphates stable under neutral to alkaline conditions were formed. As a result, the optimal synthesis conditions for obtaining pure WH under low-temperature aqueous conditions in the Ca（OH）2–Mg（OH）2–H3PO4 ternary system were determined to be Ca:Mg = 2.85:1, （Ca + Mg）/P = 1, and a pH of approximately 5.0.