  The increasing worldwide energy consumption calls for the design of more
efficient energy systems. Thermoelectrics could be used to convert waste heat
back to useful electric energy if only more efficient materials were available.
The ideal thermoelectric material combines high electrical conductivity and
thermopower with low thermal conductivity. In this regard, the intermetallic
type-I clathrates show promise with their exceedingly low lattice thermal
conductivities [1]. Here we report the successful incorporation of cerium as
guest atom into the clathrate crystal structure. In many simpler intermetallic
compounds, this rare earth element is known to lead, via the Kondo interaction,
to strong correlation phenomena including the ocurrence of giant thermopowers
at low temperatures [2]. Indeed, we observe a 50% enhancement of the
thermopower compared to a rare earth-free reference material. Importantly, this
enhancement occurs at high temperatures and we suggest that a `rattling'
enhanced Kondo interaction [3] underlies this effect.
