Factors affecting the hydriding properties of Ca_xEu_1-xNi₅ (0 ≤ x ≤ 1) compounds

Electron spectroscopy studies (Auger electron spectroscopy, X-ray photoelectron spectroscopy and UV photoemission spectroscopy) were performed on the Haucke compounds Ca_xEu_1-xNi₅ (x = 0, 0.17 and 0.75). These compounds display widely differing hydriding characteristics for the various x values, with regard both to activation kinetics and to equilibrium properties. The analysis of the valence band structures of these compounds reveals a striking similarity. Hence the differing equilibrium properties are probably not dominated by electronic contributions. Interstitial hole size calculations indicate a slight increase in the hole radii with increasing calcium content of the compound. This may account for the trend in the stabilities of the corresponding hydrides. Surface analysis of the argon-handled as-inserted samples points to the presence of very thin (i.e. a few atomic layers) passivation films on the compounds. The variation in the reactivities of the as-inserted compounds towards hydrogen (during the first activation cycle) is attributed to the variation in the permeability characteristics of these oxidized overlayers.