A misunderstood member of the nagelschmidtite family unveiled: structure of Ca5Na2(PO4)4 from X-ray powder diffraction data

During the development of procedures to synthesize carbonate hydroxyapatite with properties resembling those of biological material, a new phase was found to form after high temperature treatments in the presence of sodium. The compound was isolated and analytically characterized, leading to its identification as Ca5Na2(PO4)4: an analogous Ca–Na phosphate was previously described, but with rather poor diffraction data and uncertain stoichiometry. Accurate XRD powder patterns revealed a hexagonal cell, space group P63mc (186), a=10.64 Å, c=21.71 Å, Z=6, ρ=2.93 g/cm3, with the a axis double and the c axis triple those of the subcell typical of α-phases in the well-known system A2XO4 (or ABXO4). The complete structural determination was carried out and the relevant aspects and properties discussed.