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

Abstract

We report the synthesis and temperature-dependent structural, transport, and thermal properties of type-II clathrate K 8+xBa 16-xGa 40-ySn 96-z□ y+z (1.2 ≤ x ≤ 2.8, 2.0 ≤ y ≤ 3.3, 0.8 ≤ z ≤ 6.6, □ = 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 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∼-120 μV/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 Ba 8Ga 16Sn 30. 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. © 2011 American Physical Society.