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A thermodynamic assessment of hydrogen sorption in LaNi_5-xAl_x-H (0 ≤ x ≤ 1) alloys was performed combining experimental and theoretical investigations. The occurrence of sloped plateaux in Pressure Composition Isotherms (PCI) is discussed on the basis of compositional inhomogeneities and of possible re-distribution of metallic elements between the hydrogen rich and the hydrogen poor phases.

In order to provide input for the thermodynamic assessment, literature data were reviewed and PCI experiments were performed at increasing temperatures on alloys with different Al contents. The presence of a compositional distribution was investigated using Rietveld refinement of X-Ray diffraction patterns, volumetric and calorimetric measurements, both on as received and annealed samples with LaNi_4.8Al_0.2 nominal composition. Moreover, ab initio energies of formation for LaNi₅, LaAl₅ and the respective hydrides were calculated. A full CALPHAD assessment of the Gibbs energy for LaNi_5-xAl_xH_y (0 ≤ x ≤ 1, 0 ≤ y ≤ 7) phase was obtained, showing a good agreement between experimental and calculated results.

The presence of sloped and flat plateaux has been related to the occurrence of ortho- and paraequilibrium conditions.