Low-temperature heat-capacity study of Haucke compounds Ca${\mathrm{Ni}}_5$, Y${\mathrm{Ni}}_5$, La${\mathrm{Ni}}_5$, and Th${\mathrm{Ni}}_5$

Low-temperature heat capacities of some Haucke compounds (Ca${\mathrm{Ni}}_5$, Y${\mathrm{Ni}}_5$, La${\mathrm{Ni}}_5$, and Th${\mathrm{Ni}}_5$) were studied. The electronic specific-heat constants of these compounds are nearly the same and slightly smaller than that of pure nickel. The Deybe 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 La${\mathrm{Ni}}_5$ being the softest, followed by Ca${\mathrm{Ni}}_5$, Th${\mathrm{Ni}}_5$, and Y${\mathrm{Ni}}_5$ 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 [$3\left(g\right)\mathrm{}$ sites], and it is suggested that the Ni atoms in this plane play a critical role in governing the hydrogenation behavior of these materials.