Incommensurate Cu/Co Ordering in the TlCo2−xCuxSe2 (x∼1) System

Abstract

Cu/Co ordering in TlCo2−xCuxSe2, (x∼1) has been investigated using electron and X-ray powder diffraction. The average structure of the x=1 compound can be indexed on an F-centered orthorhombic cell, a=5.5518(6) Å, b=5.4755(5) Å, and c=13.7387(17) Å. Additional weak satellite reflections characterized by the primary modulation wave vector qi∼0.184a* and indicative of an incommensurate modulated structure are revealed by electron diffraction. Characteristic extinction conditions in conjunction with mmm Laue symmetry and known crystal chemical constraints imply a (3+1)-dimensional superspace group symmetry of Fmmm(α01)00s. The incommensurate modulation is attributed to occupational ordering of the two transition metal atoms and associated structural relaxation. The real space interpretation of the Cu/Co ordering pattern is that copper and cobalt atoms form separate layers in the structure at z=14 and 34, respectively. These layers are periodically interchanged as one moves along the x direction. Two periods of this square-wave-like modulation is close to 11a in length but is non-rational compared with the average structure.