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

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