File Name : figs1.eps Caption : fig. s1 : tem of the [0 0 1] zone in sc₅rh₆sn₁₈. (a) fourier transform of the high-resolution image. no superstructure reflections were found in this zone. (b) corresponding fourier filtered high-resolution image. (c) simulated diffraction pattern of the [0 0 1] zone. (d) electron diffraction pattern of the [0 0 1] zone. intensities appear equalized due to dynamic scattering compared to the kinematic simulation in (c). File Name : figs2.eps Caption : fig. s2 : tem of the [1 1 0] zone in sc₅rh₆sn₁₈. (a) fourier transform of the high-resolution image. no superstructure reflections were found in this zone. (b) corresponding fourier filtered high-resolution image. (c) simulated diffraction pattern of the [1 1 0] zone. (d) electron diffraction pattern of the [1 1 0] zone. intensities appear equalized due to dynamic scattering compared to the kinematic simulation in (c). File Name : figs3.eps Caption : fig. s3 first coordination sphere of the sn1-atom. a: the sc₂sn₁₄ polyhedron incorporated in trigonal prisms. b: illustration (ellipsoid probability: 90 %) of the anisotropic displacement parameters (see table s3) of all atoms forming the vertices of the polyhedron. c: sn1sc2₂sn₁₄ frank-kasper polyhedron. File Name : figs4.eps Caption : fig. s4 the wyckoff positions together with atomic coordinates and occupational factors g (not given for the fictitious structure because g = 1) for the averaged tb₅rh₆sn₁₈ structure and related to them positions in sc₅ir₆sn₁₈ type. dashed pointed lines indicate the identical positions in both structures, while dashed lines show split positions within the same space group. the fictitious structure (sg i4₁cd) was obtained for the chemical bonding analysis from the sc₅ir₆sn₁₈ type by a translationsgleiche transformation of index 2 (t2) within the group-subgroup scheme (see text for more details).