We report structural, transport, and thermal properties of carrier-tuned Ba8 Ga16 Sn30 single crystals with the type-1 clathrate structure (β phase), demonstrating that Ba8 Ga16 Sn30 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 (α phase) and of A8 Ga16 Ge30 (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°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 Å away from the centered 6d site. The temperature-linear coefficient of the specific heat is 29 mJ/mol K2 for both n - and p -type carriers in the β phase, four times larger than that for the α 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 α phase and in fact the lowest among type-1 clathrates. The lattice thermal conductivities κL for the β phase with both charge carrier types are very similar and show a glasslike temperature dependence. This behavior in κL (T) is also described using SPM analysis, where it is found that the coupling strength between guest modes and acoustic phonons for the β phase is significantly larger than that for Sr8 Ga16 Ge30.
|Physical Review B - Condensed Matter and Materials Physics
|Published - Jun 25 2008
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics