The crystal structures of Co3Se4 and Co7Se8

The crystal structures Of Co3Se4 and Co6.8Se8 have been refined from twinned single crystal and powder X-ray diffraction data. The refined structures are isostructural to the Cr3+/-xSe4 and Cr7Se8 types, respectively. The structures are described as superstructures of the Cd(OH)(2)-type structure, where the empty layers of unoccupied octahedra are replaced with alternating Co filled and empty [100] rods and Co filled Kagome nets for Co3Se4 and Co6.8Se8, respectively. The structure of Co3Se4 is not a substructure Of Co7Se8 and the path from Co7Se8 to Co3Se4 by removing 1/4 of the Co, atoms in the Kagom nets involves both gain and loss of symmetry. Yet the refinements of Co6.8Se8 suggest an extended homogeneity field. (C) 2004 Elsevier SAS. All rights reserved.