SrSn3 - a superconducting alloy with non-bonding electron pairs

SrSn3 was synthesized from the elements in a welded niobium ampoule. The cr ystal structure was determined from X-ray single crystal data. Space group R (3) over bar m, a = 6,940(2) Angstrom, c = 33,01(1) Angstrom, Z = 12, Pea rson symbol hR48. SrSn3 shows an ordered atomic distribution on four crystallographic sites. The structure is build up from two closed packed atom layers (Sn1/Sr1 and S n2/Sr2) each with the composition Sr:Sn = 1:3 and with hexagonal symmetry o f the Sr atoms. The Sn atoms are shifted with respect to the ideal position s of a closed packed layer in a way that Sn triangles, which are separated by Sr atoms, result. Translational symmetry along the c axis arises from a 12-layer stacking sequence with hexagonal and cubic closest packing motives . Due to the layer sequence ABABCACABCBC... units of three face-sharing Sn octahedra result (condensation through Sn2 atoms) which form the Sn partial structure. The octahedra chains run parallel to the c axis and are connect ed by exclusively vertex sharing Sn octahedra (Sn1 atoms). Temperature dependent susceptibility measurements reveal superconducting pr operties. LMTO band structure calculations verify the metallic behavior. An analysis of the density of states with the help of the electron localizati on function (ELF) shows, that two kinds of lone pairs occur in this interme tallic phase: non-bonding electron pairs with the shape of a sp(2) orbital hybrid are located at the Sn2 atoms and lone pairs with p orbital character are located at Sn1 atoms. The role of lone pairs with respect to the super conducting property is discussed.