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Avoided crossing of rattler modes in thermoelectric materials

Abstract

Engineering of materials with specific physical properties has recently focused on the effect of nano-sized ‘guest domains’ in a ‘host matrix’ that enable tuning of electrical, mechanical, photo-optical or thermal properties. A low thermal conductivity is a prerequisite for obtaining effective thermoelectric materials, and the challenge is to limit the conduction of heat by phonons, without simultaneously reducing the charge transport. This is named the ‘phonon glass–electron crystal’ concept and may be realized in host–guest systems. The guest entities are believed to have independent oscillations, so-called rattler modes, which scatter the acoustic phonons and reduce the thermal conductivity. We have investigated the phonon dispersion relation in the phonon glass–electron crystal material Ba8Ga16Ge30 using neutron triple-axis spectroscopy. The results disclose unambiguously the theoretically predicted avoided crossing of the rattler modes and the acoustic-phonon branches. The observed phonon lifetimes are longer than expected, and a new explanation for the low κL is provided.

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Figure 1: The clathrate type-I structure and a schematic illustration of phonons in cage structures.
Figure 2: Phonon dispersion maps measured by inelastic neutron scattering.
Figure 3: Comparison of phonon dispersion branches and constant-q scans around the region of avoided crossing.
Figure 4: Comparison of single-crystal and powder inelastic neutron scattering and Raman spectroscopy.

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Acknowledgements

This work is based on experiments carried out at the Swiss spallation neutron source SINQ, Paul Scherrer Institute, Villigen, Switzerland. N. Lock, B. L. Pedersen and T. B. S. Jensen are thanked for assistance during measurements at PSI. Support by A. Schultz and M. Miller was highly appreciated during single-crystal testing at SCD, IPNS at Argonne National Laboratory. We acknowledge the initial beamtime at BT7 supported by Y. Chen and J. Lynn at NIST Center for Neutron Research. The work was supported by the Danish Research Councils through DANSCATT. N.B.C. was supported by Swiss NSF grant 200020-105175. J.A. acknowledges support from the Swedish Research Council and the Foundation for Strategic Research.

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Correspondence to Mogens Christensen or Bo B. Iversen.

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Christensen, M., Abrahamsen, A., Christensen, N. et al. Avoided crossing of rattler modes in thermoelectric materials. Nature Mater 7, 811–815 (2008). https://doi.org/10.1038/nmat2273

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