Systematic Study of Lattice Specific Heat of Filled Skutterudites

抄録

The lattice specific heat C_lat of La-based filled skutterudites LaT₄X₁₂ (T=Fe, Ru and Os; X=P, As, and Sb) has been systematically studied, and both the Debye temperature Θ_D and the Einstein temperature Θ_E of LaT₄X₁₂ were carefully estimated. We confirmed that a correlation exists between Θ_D and the reciprocal of the square root of average atomic mass for LaT₄P₁₂, LaT₄As₁₂, and LaT₄Sb₁₂. The Θ_D of filled skutterudites was found to depend mainly on the nature of the species X forming the cage. The temperature dependence of C_lat⁄T³ for LaT₄X₁₂ exhibited a large broad maximum at low temperatures (10–30 K), which suggests a nearly dispersionless low-energy optical mode characterized by Einstein specific heat. Since no such broad maximum exists for the unfilled skutterudite RhP₃, the low-energy optical modes are associated with vibration involving La ions in the X₁₂ cage (the so-called “guest ion modes”). The Θ_E of filled skutterudites was found to roughly correspond to the energy of low-energy guest ion optical modes. Furthermore, a good correlation was shown to exist between Θ_E and r_R–X−r_R3+, where r_R–X is the R–X distance and r_R3+ is the effective ionic radius of R³⁺. As r_R–X−r_R3+ increased, Θ_E was found to decrease.