Simultaneous structure and carrier tuning of dimorphic clathrate Ba8Ga16Sn30

We report structural, transport, and thermal properties of carrier-tuned Ba8Ga16Sn30 single crystals with the type-1 clathrate structure (beta phase), demonstrating that Ba8Ga16Sn30 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 (alpha phase) and of A(8)Ga(16)Ge(30) (A=Sr,Eu). Differential thermal analysis and powder x-ray diffraction show that both phases are stable up to their virtually identical melting point of 520 +/- 3 degrees C. Refinements of single-crystal x-ray diffraction data indicate that the Ba(2) guest ion in the tetrakaidecahedron occupies the off-center 24k sites which are 0.43-0.44 angstrom 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 beta phase, four times larger than that for the alpha phase, suggesting contributions from tunneling of the Ba(2) guest ions between off-center minima. Analysis of specific heat with a soft-potential model (SPM) gives a characteristic energy of 20 K for the Ba(2) vibration, significantly lower than 50 K for the alpha phase and in fact the lowest among type-1 clathrates. The lattice thermal conductivities kappa(L) for the beta phase with both charge carrier types are very similar and show a glasslike temperature dependence. This behavior in kappa(L)(T) is also described using SPM analysis, where it is found that the coupling strength between guest modes and acoustic phonons for the beta phase is significantly larger than that for Sr8Ga16Ge30.