Thulium nickel/lithium distannide, TmNi1-x Li x Sn2 (x = 0.035).

The quaternary thulium nickel/lithium distannide, TmNi1-x Li x Sn2 (x = 0.035), crystallizes in the ortho-rhom-bic LuNiSn2 structure type. The asymmetric unit contains three Tm sites, six Sn sites, two Ni sites and one Ni/Li site [relative occupancies = 0.895 (8):0.185 (8)]. Site symmetries are .m. for all atoms. The 17-, 18- and 19-vertex distorted pseudo-Frank-Kasper polyhedra are typical for all Tm atoms. Four Sn atoms are enclosed in a 12-vertex deformed cubo-octa-hedron, and another Sn atom is enclosed in a penta-gonal prism with three added atoms. A tricapped trigonal prism is typical for a further Sn atom. The coordination number for all Ni atoms and Ni/Li statistical mixtures is 12 (fourcapped trigonal prism [Ni/LiTm5Sn5]). Tm atoms form the base of a prism and Ni/Li atoms are at the centres of the side faces of an [SnTm6Ni/Li3] prism. These isolated prisms are implemented into three-dimensional-nets built out of Sn atoms. Electronic structure calculations using TB-LMTO-ASA suggest that the Tm and Ni/Li atoms form positively charged n[TmNi/Li] (m+) polycations which compensate the negative charge of 2n[Sn] (m-) polyanions. Analysis of the inter-atomic distances and electronic structure calculations indicate the dominance of a metallic type of bonding.