Interplay between thermoelectric and structural properties of type-I clathrate K₈Ga₈Sn₃₈ single crystals

We report structural, transport, and thermal properties of type-I clathrate K₈Ga₈Sn₃₈ single crystals grown by the self-flux method. Single-crystal x-ray diffraction analysis confirmed that the guest K⁺ ion locates on the center in the tetrakaidecahedron composed of Ga and Sn atoms. The thermopower is largely negative, -200μV/K at room temperature, irrespective of the flux used during growth (Ga or Sn). The thermal conductivity κ (T) exhibits a large peak at 14 K. These observations in K8 Ga8 Sn38 are contrasting with the splitting of the guest site and the glasslike behavior in κ (T) reported for type-I Ba₈Ga ₁₆Sn₃₀ although the free space for the guest is almost the same in both compounds. The electrostatic potential for the tetrakaidecahedron was calculated using the occupation probabilities of Ga ions in the three sites on the cage. It is found that the off-centered state is stabilized for the Ba²⁺ ions in Ba₈Ga₁₆Sn₃₀ by the partial occupation of Ga anion in the 16i site while the on-center state for K₈Ga₈Sn₃₈ is stabilized by the strongly preferred occupation of Ga anions in the 6c site. We conclude that the charge distribution on the cage is crucial for the splitting of the guest site into off-center positions in the tetrakaidecahedron of the type-I clathrate.