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

Abstract

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 [$3(g)$ sites], and it is suggested that the Ni atoms in this plane play a critical role in governing the hydrogenation behavior of these materials.