Novel thermoelectrics emerging from multinary freedoms of clathrates encapsulating rare-earths and alkaline metals

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Intermetallic clathrates have high potential as thermoelectric converters because of the glass-like thermal conductivity in spite of the three dimensional crystal structure. To understand the mechanism of the reduced thermal conductivity, we have studied type-I clathrates encapsulating alkaline-earth and rare-earth atoms by means of structural refinement, thermal, ultrasonic, Raman scattering and photoemission measurements. It is revealed that the guest in a relaxed space inside the cage can rotate among off-center sites and this motion strongly scatters acoustic phonons to reduce the thermal conductivity.