Calcium orthophosphates (CaPs) are important in geology, biomineralization, animal metabolism and biomedicine, and constitute a structurally and chemically diverse class of minerals. In the case of dicalcium phosphates, ever since brushite (CaHPO₄·2H₂O, dicalcium phosphate dihydrate, DCPD) and monetite (CaHPO₄, dicalcium phosphate, DCP) were first described in 19^th century, the form with intermediary chemical formula CaHPO₄·H₂O (dicalcium phosphate monohydrate, DCPM) has remained elusive. Here, we report the synthesis and crystal structure determination of DCPM. This form of CaP is found to crystallize from amorphous calcium hydrogen phosphate (ACHP) in water-poor environments. The crystal structure of DCPM is determined to show a layered structure with a monoclinic symmetry. DCPM is metastable in water, but can be stabilized by organics, and has a higher alkalinity than DCP and DCPD. This study serves as an inspiration for the future exploration of DCPM’s potential role in biomineralization, or biomedical applications.

正磷酸钙（CaPs）在地质，生物矿化，动物代谢和生物医学中很重要，并且构成了结构和化学上多样化的矿物质。在磷酸二钙，由于透钙磷石过的情况下（使用CaHPO ₄ ·2H ₂ O，磷酸二钙二水合物，DCPD）和斜磷钙石（使用CaHPO ₄，磷酸二钙，DCP）首先在19描述的^日世纪以来，随着中介化学式使用CaHPO形式₄ ·H ₂O（磷酸二氢钙一水合物，DCPM）仍然难以捉摸。在这里，我们报告DCPM的合成和晶体结构的确定。发现在贫水环境中，这种形式的CaP从无定形磷酸氢钙（ACHP）结晶。确定DCPM的晶体结构以显示具有单斜晶对称性的层状结构。DCPM在水中是亚稳态的，但可以被有机物稳定，并且比DCP和DCPD具有更高的碱度。这项研究为DCPM在生物矿化或生物医学应用中的潜在作用的未来探索提供了启发。