Thermodynamic properties of non-stoichiometric LaNix-1Cu-H systems

A series of non-stoichiometric LaNix-1Cu intermetallic compounds with x = 5.0 to 6.0 has been prepared and characterized by X-ray diffraction and shown to be single phase. The thermodynamics of hydrogen absorption and desorption by these compounds have been determined from their equilibrium hydrogen pressures and from the calorimetric measurement of their heats of reaction with H-2(g). For x > 5.4 the system appears to be above its critical composition for the co-existence of the dilute and hydride phases, in agreement with earlier results obtained from in situ X-ray diffraction. As x increases from 5.0 to 6.0, the hydrogen pressures corresponding to the plateaux, or else to the ''plateau-like'' regions, increase and the hydrogen capacities decrease. For the plateaux and plateau-like regions the calorimetrically measured enthalpies of reaction are found to increase in magnitude with x, whereas the corresponding entropy changes are nearly independent of x, and hence the enthalpy changes cause the observed increase of p(H2). The hysteresis decreases with increasing x.