SrSn4: a superconducting stannide with localized and delocalized bond character.

The title compound is the tin-richest phase in the system Sr-Sn and is obtained by stoichiometric combination of the elements. SrSn(4) peritecticly decomposes under formation of SrSn(3) and Sn at 340 degrees C. The structure determined from a single crystal shows a new structure type with a novel structure motive in tin chemistry. It can be described by a corrugated, distorted quadratic net of tin atoms as the only building unit. The nets intersect at common Sn atoms, and the resulting channels host the Sr atoms. The structure can alternatively be described as an intergrowth structure of the AlB(2)-type and W-type. The atoms that are connected by the two shortest Sn-Sn distances (2.900 and 3.044 A) form a two-dimensional net consisting of hexagons of tin atoms. The hexagons have boat conformation in contrast to the rather similar alpha-As structure type, where hexagons have a chair conformation. Further tin atoms connect the two-dimensional net of Sn hexagons. Temperature-dependent magnetic susceptibility measurements show that SrSn(4) is superconducting with T(c) = 4.8 K at 10 G. LMTO band structure and density of states calculations verify the metallic behavior of SrSn(4). An analysis of the electronic structure with the help of the electron localization function (ELF) shows that localized covalent bonds beside delocalized bonds coexist in SrSn(4).