The alluaudite-type crystal structures of Na2(Fe/Co)2Co(VO4)3 and Ag2(Fe/Co)2Co(VO4)3

Single crystals of the title compounds, disodium di(cobalt/iron) cobalt tris­(orthovanadate), Na2(Fe/Co)2Co(VO4)3, and disilver di(cobalt/iron) cobalt tris­(orthovanadate), Ag2(Fe/Co)2Co(VO4)3, were grown from a melt consisting of stoichiometric mixtures of three metallic cation precursors and vanadium pentoxide. The difficulty to distinguish between cobalt and iron by using X-ray diffraction alone forced us to explore several models, assuming an oxidation state of +II for Co and +III for Fe and a partial cationic disorder in the Wyckoff site 8f containing a mixture of Co and Fe with a statistical distribution for the Na compound and an occupancy ratio of 0.4875:0.5125 (Co:Fe) for the Ag compound. The alluaudite-type structure is made up from [10-1] chains of [(Co,Fe)2O10] double octa­hedra linked by highly distorted [CoO6] octa­hedra via a common edge. The chains are linked through VO4 tetra­hedra, forming polyhedral sheets perpendicular to [010]. The stacking of the sheets defines two types of channels parallel to [001] where the Na+ cations (both with full occupancy) or Ag+ cations (one with occupancy 0.97) are located.