Ordering of vacancies in type-I tin clathrate: superstructure of Rb8Sn44 square2.

One important step toward the understanding of the mechanisms of thermoelectric properties is the knowledge about local distortions and vacancy ordering in clathrates. The type-I clathrate Rb8Sn44 shows in single-crystal and powder diffraction patterns a 2 x 2 x 2 supercell of the primitive cubic unit cell (Pmn), which originates from an ordering of the partially occupied site. The latter is distributed around a 41 screw axis, and the vacancies lead to a relaxation of the tin framework, thereby creating a local distortion of the tetrakaidecahedron.