Thermodynamics of hydrogen trapping in intermetallic compounds: Application to [formula omitted

Abstract

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 H 2 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 H 2 in the α-phase of the LaNi 5 H system are given at 298.2 K. These have been determined from calorimetric determinations of ΔH H and measurements of ΔG H ( = RT ln P 1 2 H 2 ) . 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.