The chemical compound Na(Na,Mn)7Mn22(PO4)18·0.5H2O was synthesized with a Tuttle pressure vessel at 800°C and 1 kbar during experimental investigations on the conditions of fillowite formation. The crystal structure of a single-crystal, space group R3̅ (No. 148), a = 15.2741(9), c = 43.334(3) Å, Z = 6, was determined from X-ray intensity data and refined up to R1 = 0.0728 and R1{FO>4σ(FO)} = 0.0546. The crystal structure of Na(Na,Mn)7Mn22(PO4)18·0.5H2O is similar to the structure of fillowite, Na6Ca3(Mn,Fe)21(PO4)18., but a significant difference between the synthetic H2O containing compound and fillowite is the replacement of (Na12)O8- by Mn(H2O)2O5- polyhedra, which form characteristic six-member rings. The coordination polyhedra around Na12 and Mn12 are so different that simple isomorphous substitution of Na12 by Mn12 is very unlikely, but whole six-member rings either of (Na12)O8- or (Mn12)(H2O)2O5 polyhedra seem to be distributed statistically over the crystal structure. No indication for ordering, e.g. sheet by sheet, has been observed. Na(Na,Mn)7Mn22(PO4)18·0.5H2O is the first fillowite-like compound for which water is located in the structure from single-crystal study. This result is proved by infrared spectral data and indicates that water can play an essential role in the fillowite structure-type. Structural features of the synthetic compounds Na(Na,Mn)7Mn22(PO4)18·0.5H2O, Na4Ca4Mg21(PO4)18 and fillowite are compared.

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