Systematic Study of Lattice Specific Heat of Filled Skutterudites

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The lattice specific heat $C_{\text{lat}}$ of La-based filled skutterudites LaT4X12 ($T = \text{Fe}$, Ru and Os; $X = \text{P}$, As, and Sb) has been systematically studied, and both the Debye temperature $\Theta_{\text{D}}$ and the Einstein temperature $\Theta_{\text{E}}$ of LaT4X12 were carefully estimated. We confirmed that a correlation exists between $\Theta_{\text{D}}$ and the reciprocal of the square root of average atomic mass for LaT4P12, LaT4As12, and LaT4Sb12. The $\Theta_{\text{D}}$ of filled skutterudites was found to depend mainly on the nature of the species $X$ forming the cage. The temperature dependence of $C_{\text{lat}}/T^{3}$ 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 $X_{12}$ cage (the so-called “guest ion modes”). The $\Theta_{\text{E}}$ 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 $\Theta_{\text{E}}$ and $r_{\text{R--X}}-r_{\text{R3+}}$, where $r_{\text{R--X}}$ is the $R$–$X$ distance and $r_{\text{R3+}}$ is the effective ionic radius of $R^{3+}$. As $r_{\text{R--X}}-r_{\text{R3+}}$ increased, $\Theta_{\text{E}}$ was found to decrease.
2009-12-15

Systematic Study of Lattice Specific Heat of Filled Skutterudites

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