Microstructural study of copper‐intercalated niobium disulphide and tantalum disulphide Cu_xNbS₂ and Cu_xTaS₂ II. Order‐disorder transitions and incommensurate superstructures in Cu_xNbS₂ (0.8 < x < 1)

The intercalation compound Cu_xNbS₂ with copper concentrations in the range x = 0.8 to 0.9 is studied by electron diffraction and transmission microscopy. Particular attention is paid to the structural aspects of the thermal behaviour of these compounds in which the copper intercalate behaves somewhat like a two dimensional liquid at temperatures above the order‐disorder transition at 50 °C. From electron diffraction analysis it is deduced that upon cooling from above this temperature where diffuse rings and lines in the diffraction pattern are associated with the partial order of the intercalate within the van der Waals gap, successive steps of ordering occur. In a first step the ordering of copper ion‐vacancy arrays along rows in the close packed directions gives rise to commensurate and incommensurate superstructures each in three possible orientations with respect to the host lattice. Upon further cooling diffuse streaks resolving gradually into spots present evidence for further order in a lateral direction with respect to the vacancy‐intercalate rows. Increased correlation finally gives rise to a two‐dimensional modulated superstructure.