We report structural, transport, and thermal properties of type-I clathrate ${\text{K}}_8{\text{Ga}}_8{\text{Sn}}_{38}$ single crystals grown by the self-flux method. Single-crystal x-ray diffraction analysis confirmed that the guest ${\text{K}}^{+}$ ion locates on the center in the tetrakaidecahedron composed of Ga and Sn atoms. The thermopower is largely negative, $-200\mu \text{V}/\text{K}$ at room temperature, irrespective of the flux used during growth (Ga or Sn). The thermal conductivity $\kappa \left(T\right)$ exhibits a large peak at 14 K. These observations in ${\text{K}}_8{\text{Ga}}_8{\text{Sn}}_{38}$ are contrasting with the splitting of the guest site and the glasslike behavior in $\kappa \left(T\right)$ reported for type-I ${\text{Ba}}_8{\text{Ga}}_{16}{\text{Sn}}_{30}$ 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 ${\text{Ba}}^{2+}$ ions in ${\text{Ba}}_8{\text{Ga}}_{16}{\text{Sn}}_{30}$ by the partial occupation of Ga anion in the $16i$ site while the on-center state for ${\text{K}}_8{\text{Ga}}_8{\text{Sn}}_{38}$ 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.

• Received 8 February 2010

DOI:https://doi.org/10.1103/PhysRevB.81.165110