Cs2.91Na1.34Fe3+0.25[Fe3O(SO4)6(H2O)3]•5H2O, a member of the Maus's salt family

The title compound, tricaesium sodium iron(III) mu(3)-oxido-hexa-mu(2)-sulfato-tris[aquairon(III)] pentahydrate, Cs2.91Na1.34Fe0.253+[Fe3O(SO4)(6)(H2O)(3)]center dot 5H(2)O, belongs to the family of Maus's salts, K-5[Fe3O(SO4)(6)(H2O)(3)]center dot 6H(2)O, which is based on the triaqua-mu 3-oxido-hexa-mu-sulfato-triferrate(III) anion, [Fe3O(SO4)(6)(H2O)(3)](5-), with Fe in a characteristically distorted octahedral coordination environment, sharing a common corner via an oxide O atom. Cs in four different cation sites, Na in three different cation sites and five water molecules link the anions in three dimensions and set up a crystal structure in which those parts parallel to (001) and within 0.05 < z < 0.95 have a distinct trigonal pseudosymmetry, whereas the cation arrangement and bonding near z similar to 0 generate a clear-cut noncentrosymmetric polar edifice with the monoclinic space group C2. The structure shows some cation disorder in the region near z similar to 1/2, where one Na atom in octahedral coordination is partly substituted by Fe3+, and a Cs atom is substituted by small amounts of Na on a separate nearby site. One Na atom, located on a twofold axis at z = 0 and tetrahedrally coordinated by four sulfate O atoms of two [Fe3O(SO4)(6)(H2O)(3)](5-) units, plays a key role in generating the noncentrosymmetric structure. Three of the seven different cation sites are on twofold axes (one Na+ site and two Cs+ sites), and all other atoms of the structure are in general positions.