Localized vibrational modes in metallic solids

Abstract

Filled skutterudite antimonides1,2 are cubic compounds with the formula RM4Sb12, where R is a rare-earth element (such as La or Ce), and M is a transition metal (for example, Fe or Co). The rare-earth ion is weakly bound in an oversized atomic cage formed by the other atoms. Its presence has been shown to cause a dramatic reduction in the lattice component of the thermal conductivity, while having little effect on the electronic properties3,4,5 of the compound. This combination of properties makes filled skutterudites of interest as thermoelectric materials. It has been suggested4 that localized, incoherent vibrations of the rare-earth ion are responsible for the reduction in thermal conductivity, but no direct evidence for these local vibrational modes exists. Here we report the observation of local modes in La-filled skutterudites, using heat capacity, elastic constant and inelastic neutron scattering measurements. The La atoms show unusual thermodynamic behaviour, characterized by the presence of two low-energy localized modes. Our results suggest that consideration of local modes will play an important role in the design of the next generation of thermoelectric materials.

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Figure 1: Specific heat divided by temperature versus temperature squared for La0.9Fe3CoSb12 and CoSb3.
Figure 2: Elastic moduli c11 and c44 for La0.75Fe3CoSb12 as a function of temperature.
Figure 3: Difference in the inelastic neutron scattering data between LaFe4Sb12 and CeFe4Sb12 versus energy loss.

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Acknowledgements

We thank J. Feldman and R. Leisure for discussions, and A. Migliori and T. Darling for assistance with the RUS measurements. This work was supported by the Division of Materials Sciences, US Department of Energy, a Cooperative Research and Development Agreement with Marlow Industries, and the NSF.

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Correspondence to B. C. Chakoumakos.

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Keppens, V., Mandrus, D., Sales, B. et al. Localized vibrational modes in metallic solids. Nature 395, 876–878 (1998). https://doi.org/10.1038/27625

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