Microstructural study of copper-intercalated niobium disulphide and tantalum disulphide CuxNbS2 and CuxTaS2. I. Phase Transitions in Cu0.5NbS2 and Cu0.5TaS2

Abstract

The phase transitions in the copper intercalated transition metal dichalcogenides Cu0.5NbS2 and Cu0.5TaS2 are studied by electron microscopy and diffraction. These materials are found to undergo upon cooling two distinct types of phase transitions. One is an order—disorder type of transition slightly below room temperature (γ → β), which is accompanied by a particular configuration of diffuse scattering in the short range order state evolving into a distinct superstructure pattern at temperatures below 240 K. The microstructure is characterized by a domain structure exhibiting orientation as well as translation variants. At still lower temperatures a “continuous transformation” of displacive nature (β → α) occurs whereby the host lattice becomes gradually modulated with the period of the intercalate. No further fragmentation into domains occurs. These transitions can be described in terms of a binary system (copper-vacancies) undergoing a solid-liquid transition.