Thermoelectric materials are of interest for applications in active cooling and in waste heat recovery. Their ability to perform well in these processes is benchmarked by the thermoelectric figure of merit

\(Z=\left( \sigma/\kappa\right)S^2\)

where \(\sigma\) is the electrical conductivity, \(\kappa\) the thermal conductivity, and S the thermopower. We are particularly interested in the class of intermetallic clathrates. These are solids made up of a three-dimensional network of covalently bonded „host“ atoms forming large cavities that are filled by mostly non-covalently bonded „guest“ atoms. The vibrations of the guest atoms within the cages („rattling“ or Einstein modes) are held responsible for the ultralow thermal conductivities of these materials.

Fig.1: Cartoon of the physics of rattling Ce atoms, boosting the Kondo effect [from Prokofiev et al., Nat. Mater. 12, 1096 (2013)]

Our current investigations focus on:

• The synthesis of high-quality single crystal series, to systematically study the composition/structure/property relationship. We have succeeded to incorporate sizable amounts of the typical Kondo element Ce into the clathrate cages, something that had long remained elusive.
• The investigation of the phonon spectra by inelastic neutron and x-ray scattering.
• High-accuracy 3\(\omega\) thermal conductivity measurements that allow for a detailed data analysis
• We have also investigated the effects of nanostructuring, using both top-down techniques and rapid cooling by melt spinning as bottom up technique.

Recent key discoveries include:

• The observation that low-lying phonon modes strongly interact with acoustic phonon modes.
• The discovery of a boosting of the spin Kondo effect via the coupling of the rattling mode to the 4f electron of Ce.
• The demonstration of a Kondo-like strong interaction between the ratting (local) and acoustic (propagating) phonons.

Fig.2.: Kondo-like phonon thermal conductivity attributed to the scattering of acoustic phonons from rattling modes [from Ikeda et al., Nat. Commun. 10, 887 (2019)]

For details, see the list of publications or contact us.