Low-temperature heat-capacity study of Haucke compounds CaNi5, YNi5, LaNi5, and ThNi5

Low-temperature heat capacities of some Haucke compounds (CaNi5, YNi5, LaNi5, and ThNi5) were studied. The electronic specific-heat constants of these compounds are nearly the same and slightly smaller than that of pure nickel. The Debye temperatures of these compounds show a wider variation than the electronic specific-heat constants. It is found that the lattice rigidity of these four compounds varies considerably, with that of LaNi5 being the softest, followed by CaNi5, ThNi5, and YNi5 in order of increasing rigidity. This appears to influence their hydrogen absorption capacity. The relative softness is correlated with the Ni--Ni distances in the midplane [3(g) sites], and it is suggested that the Ni atoms in this plane play a critical role in governing the hydrogenation behavior of these materials