The missing representatives of the hydrated sodium orthophosphate phases: Na ₃ (PO ₄ )(H ₂ O) ₇ and Na ₃ (PO ₄ )(H ₂ O) ₆

The crystal structures of the long-known compound Na₃(PO₄)(H₂O)₆, tris-odium orthophosphate hexa-hydrate, and the compound Na₃(PO₄)(H₂O)₇, tris-odium orthophosphate hepta-hydrate, the possible existence of which is discussed in the literature, were elucidated by single-crystal X-ray diffraction. In both crystal structures, all the water mol-ecules are bound to the sodium cations, but the different water content leads to different arrangements in terms of polyhedral linkage. In the case of the hepta-hydrate (space group <i>Pca</i>2₁, <i>Z</i> = 4), this results in a layered structure made up from three sixfold coordinated Na⁺ cations with phosphate units in between. In the case of the hexa-hydrate (space group <i>P</i>1, <i>Z</i> = 4), a three-dimensional network is realised by one fivefold and five sixfold coordinated Na⁺ cations, in which the phosphate units are embedded in the voids. In both crystal structures, the water mol-ecules are involved in complex O-H⋯O hydrogen-bonding networks and form moderately strong hydrogen bonds on average, almost exclusively with the phosphate O atoms. It is noteworthy that some O atoms accept up to five such bonds.