Thermodynamic properties of the f-electron-nuclear hyperfine-coupled composite multiplets of Pr ions in filled-skutterudite PrRu₄P ₁₂

It has recently been found that composite multiplets are formed by the hyperfine coupling between the triplet crystalline-electric-field ground state of the 4f electrons and the ¹⁴¹Pr nuclear spin in Pr ions of the filled skutterudite PrRu4P12. Entropy analysis of the characteristic Schottky-type peak structure appearing in the low-temperature specific heat demonstrates that it is actually caused by thermal excitations of not only 4f electrons but also ¹⁴¹Pr nuclei. The hyperfine coupling constant for Pr ions in nonmetallic PrRu4P12 is determined to be A +0.050(2) K, which is almost the same as in metallic Pr-based compounds. This fact corroborates that the intra-ion hyperfine coupling of Pr ions is independent of the type of material.