We report structural, transport, and thermal properties of carrier-tuned Ba 8 Ga 16 Sn 30 single crystals with the type-1 clathrate structure ͑␤ phase͒, demonstrating that Ba 8 Ga 16 Sn 30 is a unique thermoelectric clathrate material wherein both the structure type and the carrier type are tunable. The results are compared with the properties of the better known type-8 structure ͑␣ phase͒ and of A 8 Ga 16 Ge 30 ͑A =Sr,Eu͒. Differential thermal analysis and powder x-ray diffraction show that both phases are stable up to their virtually identical melting point of 520Ϯ 3°C. Refinements of single-crystal x-ray diffraction data indicate that the Ba͑2͒ guest ion in the tetrakaidecahedron occupies the off-center 24k sites which are 0.43-0.44 Å away from the centered 6d site. The temperature-linear coefficient of the specific heat is 29 mJ/ mol K 2 for both n-and p-type carriers in the ␤ phase, four times larger than that for the ␣ phase, suggesting contributions from tunneling of the Ba͑2͒ guest ions between off-center minima. Analysis of specific heat with a soft-potential model ͑SPM͒ gives a characteristic energy of 20 K for the Ba͑2͒ vibration, significantly lower than 50 K for the ␣ phase and in fact the lowest among type-1 clathrates. The lattice thermal conductivities L for the ␤ phase with both charge carrier types are very similar and show a glasslike temperature dependence. This behavior in L ͑T͒ is also described using SPM analysis, where it is found that the coupling strength between guest modes and acoustic phonons for the ␤ phase is significantly larger than that for Sr 8 Ga 16 Ge 30 .

The present controversy over the origin of glasslike thermal conductivity observed in certain crystalline materials is addressed by studies on single-crystal x-ray diffraction, thermal conductivity κ(T ) and specific heat Cp(T ) of carrier-tuned Ba8Ga16X30 (X = Ge, Sn) clathrates. These crystals show radically different low-temperature κ(T ) behaviors depending on whether their charge carriers are electrons or holes, displaying the usual crystalline peak in the former case and an anomalous glasslike plateau in the latter. In contrast, Cp(T ) above 4 K and the general structural properties are essentially insensitive to carrier tuning. We analyze these combined results within the framework of a Tunneling/Resonant/Rayleigh scatterings model, and conclude that the evolution from crystalline to glasslike κ(T ) is accompanied by an increase both in the effective density of tunnelling states and in the resonant scattering level, while neither one of these contributions can solely account for the observed changes in the full temperature range. This suggests that the most relevant factor which determines crystalline or glasslike behavior is the coupling strength between the guest vibrational modes and the frameworks with different charge carriers.

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 Ba8Ga16Sn30 single crystals grown in the type-I clathrate structure (β-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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