The stability of intercalation compounds AgxTaSs

The electrochemical properties of intercalates AgxTaS2 prepared by high-temperature techniques were measured in a solid electrolyte (AgI) electrochemical cell. The EMF versus x curve at 444 K showed the presence of stage-2 and −1 phases for 0.22<x<0.35 and 0.55<x<0.68 respectively. From X-ray powder diffraction evidence was found that for x<0.1 a disordered staged phase, presumably a third stage phase is stable. The EMF is small (less about 0.25 V) and the temperature coefficient of the EMF is positive. A simple model was used to explain the observed EMF versus x curve. The free enthalpy of intercalation of the stage-1, stage-2 and stage-3 intercalates is expressed into (1) a binding energy of the silver atoms to the lattice, proportional to x; (2) a repulsion energy of silver atoms in a plane and between silver atoms in neighbouring planes; (3) an elastic energy needed to open a gap between sandwiches using the rigid plate model; (4) the configurational part of the entropy using the mean field approximation. Using the Einstein model of vibration for silver atoms one finds a vibrational part of the intercalation entropy ΔSv which is proportional to x; in the free enthalpy of intercalation the contribution TΔSv can be absorbed in With five parameters the observed EMF versus x curve is reasonably well reproduced. One also finds that the difference in free enthalpy for n-staged phases with n > 2 and a two phase mixture of stage 22HTaS2 is very small which explains why higher-staged phases are difficult to obtain.