Crystal structure of (Na0.70)(Na0.70Mn0.30)(Fe3+Fe2+)2Fe2+(VO4)3 a sodium- iron- and manganese-based vanadate with the alluaudite-type structure

摘要

The title compound, sodium (sodium,manganese) triiron(II,III) tris[vana­date(V)], (Na0.70)(Na0.70,Mn0.30)(Fe³⁺,Fe²⁺)2Fe²⁺(VO4)3, was prepared by solid-state reactions. It crystallizes in an alluaudite-like structure, characterized by a partial cationic disorder. In the structure, four of the 12 sites in the asymmetric unit are located on special positions, three on a twofold rotation axis (Wyckoff position 4e) and one on an inversion centre (4b). Two sites on the twofold rotation axis are entirely filled by Fe²⁺ and V⁵⁺, whereas the third site has a partial occupancy of 70% by Na⁺. The site on the inversion centre is occupied by Na⁺ and Mn²⁺ cations in a 0.7:0.3 ratio. The remaining Fe²⁺ and Fe³⁺ atoms are statistically distributed on a general position. The three-dimensional framework of this structure is made up of kinked chains of edge-sharing [FeO6] octa­hedra stacked parallel to [10-1]. These chains are held together by VO4 tetra­hedral groups, forming polyhedral sheets perpendicular to [010]. Within this framework, two types of channels extending along [001] are present. One is occupied by (Na⁺/Mn²⁺) while the second is partially occupied by Na⁺. The mixed site containing (Na⁺/Mn²⁺) has an octa­hedral coordination sphere, while the Na⁺ cations in the second channel are coordinated by eight O atoms.