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Nature Materials 7, 811–815 (1 October 2008) | doi:10.1038/nmat2273

Avoided crossing of rattler modes in thermoelectric materials

Mogens Christensen , Asger B. Abrahamsen , Niels B. Christensen , Fanni Juranyi , Niels H. Andersen , Kim Lefmann , Jakob Andreasson , Christian R. H. Bahl & Bo B. Iversen

Engineering of materials with specific physical properties has recently focused on the effect of nano-sized |[lsquo]|guest domains|[rsquo]| in a |[lsquo]|host matrix|[rsquo]| 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 |[lsquo]|phonon glass–electron crystal|[rsquo]| 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.