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

Author: Keller Paul   Hatert Frédéric   Lissner Falk   Schleid Thomas   Fransolet André-Mathieu  

Publisher: E. Schweizerbart'sche Verlagsbuchhandlung

ISSN: 0935-1221

Source: European Journal of Mineralogy, Vol.18, Iss.6, 2006-11, pp. : 765-774

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Abstract

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 R3 (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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