Synthesis and crystal structure of Na4Ni7(AsO4)6.

The title compound, tetra-sodium hepta-nickel hexa-arsenate, was obtained by ceramic synthesis and crystallizes in the monoclinic space group C2/m. The asymmetric unit contains seven Ni atoms of which two have site symmetry 2/m and three site symmetry 2, four As atoms of which two have site symmetry m and two site symmetry 2, three Na atoms of which two have site symmetry 2, and fifteen O atoms of which four have site symmetry m. The structure of Na4Ni7(AsO4)6 is made of layers of Ni octa-hedra and As tetra-hedra assembled in sheets parallel to the bc plane. These layers are inter-connected by corner-sharing between NiO6 octa-hedra and AsO4 tetra-hedra. This linkage creates tunnels running along the c axis in which the Na atoms are located. This arrangement is similar to the one observed in Na4Ni7(PO4)6, but the layers of the two compounds are slightly different because of the disorder of one of the Ni sites in the structure of the title compound.