Off-center rattling and thermoelectric properties of type-II clathrate (K, Ba)(24)(Ga, Sn, square)(136) single crystals

Mano Satomi

Onimaru Takahiro

Yamanaka Shoji

Takabatake Toshiro

Physical Review B

We report the synthesis and temperature-dependent structural, transport, and thermal properties of type-II clathrate K8+xBa16-xGa40-ySn96-z square(y+z) (1.2 <= x <= 2.8, 2.0 <= y <= 3.3, 0.8 <= z <= 6.6, square = framework vacancy). Single-crystal x-ray diffraction analysis reveals that the guest K+ and Ba2+ ions are preferentially incorporated into the hexakaidecahedral cages and dodecahedral cages, respectively. The guest site in the former splits into four sites 0.67 angstrom away from the center to the 32e site of the cage. The splitting is consistent with the presence of four minima in the electrostatic potential in the hexakaidecahedron. The thermopower is negative and relatively large, -50 similar to -120 mu V/K at 300 K, indicating that the dominant charge carriers are electrons. The thermal conductivity displays a glasslike behavior with a plateau at around 20 K. The analysis of the specific heat indicates that the motion of the K+ ion in the hexakaidecahedron can be described by the soft-potential model, including the tunneling term. The characteristic energy of 21 K for the soft mode is as low as that of the off-center rattling of the Ba2+ ion in type-I clathrate Ba8Ga16Sn30. The present result on the type-II clathrate verifies the idea that the low-energy off-center rattling in oversized cages of intermetallic clathrates couples to the acoustic phonons to lead to the glasslike thermal conductivity.

Physics [ 420 ]

eng

American Physical Society

(c) 2011 American Physical Society

[ISSN] 1098-0121

[DOI] 10.1103/PhysRevB.84.214101

[NCID] AA11187113

[DOI] http://dx.doi.org/10.1103/PhysRevB.84.214101