For the past decade, intermetallic clathrates have been rattling their way into mainstream research in thermoelectrics. The unusual vibrations of their guest ions inside oversized cages interfere with the cage phonons while leaving the electronic flow intact, providing an exotic way to achieve a best-of-both-worlds scenario in terms of electrical and thermal transport in the same material. Here, we present the structural and thermoelectric properties of Ba8 Ga16 Sn30 single crystals grown in the type-I clathrate structure (ß-BGS), showing one of the lowest recorded thermal conductivities κ (T) for any bulk compound, while still behaving electronically as a heavily doped n -type semiconducting crystal.
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