FERMI ENERGY EFFECTS ON ABSORPTION OF HYDROGEN IN LANI5 AND LACO5 ALLOYS

The electronic state of hydrogen in LaNi5 and LaCo5 is different from that in Pd in that lower PI states are entered by electrons from the F ermi level of the alloy, emptying rather than filling the conduction b and. However, the successive lowering of the Fermi level occurring her e will contribute to a rise of the chemical potential of the dissolved hydrogen in much the same way as in palladium. In the case of LaNi5, this Fermi energy contribution cannot be separated from the elastic in teraction, which predominates in the whale concentration range and lea ds to the: formation of several two-phase regions. Besides the elastic interaction (and the electronic contribution hidden beneath it), the availability of multiple interstitial sites and the blocking effect be tween some densely packed sites contribute to the thermodynamics of th e system. In the case of LaCo5, an abrupt rise of the chemical potenti al of 6.3 kJ molH(-1) is observed at the composition of LaCo5H3, well within the range of occupation of 4e sites. It is suggested that the r ise at this perfectly stoichiometric composition is related to the ban d structure of the alloy.