Ordering of vacancies in type-I tin clathrate: Superstructure of Rb ₈Sn₄₄□₂

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 × 2 × 2 supercell of the primitive cubic unit cell (Pm3n), 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.