Thermodynamics of hydrogen trapping in intermetallic compounds: Application to LaNi5H

It has been observed for several intermetallic compound/H systems that deviations from Sieverts' law of ideal solubility occur which are in the opposite direction from the deviations from ideality which occur in pure metal/H systems. The principal source of nonideality on both types of interstitial solutions arises from the changes of the relative partial molar enthalpy of H₂ solution, ΔH_H, with H content. The variations of ΔH_H with H content are, however, in the opposite directions for intermetallic/H and pure metal/H interstitial solutions. Relative partial molar entropies for solution of H₂ in the α-phase of the $\text{LaNi}{}_5\text{H}$ system are given at 298.2 K. These have been determined from calorimetric determinations of ΔH_H and measurements of . The partial entropies show unusual extrema as a function of H content. A model based on trapping of H atoms is proposed which explains the variations of ΔH_H and ΔS_H. Since the deviations from ideality arising from an increase of ΔH_H with H content seem to be limited to solids with at least two different ordered metal components, e.g., intermetallic compounds, it seems reasonable to assign the trapping sites to interstices which result from an interchange of metal atoms. This interchange may create interstices rich in those nearest-neighbor metal atoms which have the greatest affinity for H.