Hydrothermal synthesis and crystal structure of Na(Na,Mn)7Mn22(PO4)18·0.5H2O, a new compound of fillowite structure type

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 R_{1{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.