Influence of the lattice and electronic factors on the hydrogenation properties of the RNi 5-base (R is a rare earth) haucke compounds: Results of low temperature heat capacity measurements

Abstract

The low temperature heat capacities (1 – 20 K) and the hydrogen absorption properties of some ternary Haucke compounds — LaNi 5− x Al x (0 ⩽ x ⩽ 1.5), LaNi 5−xPt x (0 ⩽ x ⩽ 5) and La 1−xTh x Ni 5 (0 ⩽ x ⩽ 1)—were investigated to examine any correlation between the ability of these compounds to absorb hydrogen and their physical properties, e.g. their electronic structure and elastic properties. The density of states at the Fermi level has a minimum near x = 1 in LaNi 5− x Al x . The electronic specific heat constants of LaNi 5−xPt x decrease in a smooth fashion as the concentration of nickel decreases. La 1−xTh xNi 5 alloys, however, only exhibit a slight change in their electronic specific heat constants. The low temperature heat capacities were also measured for the hydrides of these compounds. The electronic specific heat constant γ and the Debye temperature θ D of the LaNi 5− x Al x H y phase do not change as drastically, relative to the corresponding LaNi 5− x Al x phase, as those for the RH 2 compounds change relative to the corresponding pure metal R. This is probably caused by filling of the new bonding states formed in these compound hydrides with the added electrons of hydrogen.