SrCo2Sn8 and BaCo2Sn8: Tin-rich Stannides with Distorted SnSn6 Octahedra within Three-dimensional [Co2Sn8] Networks

Abstract

The tin-rich stannides SrCo2Sn8 and BaCo2Sn8 were synthesized from the elements in sealed tantalum tubes. They crystallize with a new structure type, space group Cccm with a=1006.0(3), b=1514.4(6), c=1385.0(6) pm for SrCo2Sn8 and a=1032.8(2), b=1516.8(3), c=1405.1(3) pm for BaCo2Sn8. The structure of the barium compound was refined on the basis of single-crystal Xray diffractometer data: wR2=0.0450, 1715 F2 values, 57 variables. The cobalt atoms have seven nearest tin neighbors with Co-Sn distances ranging from 257 to 273 pm. These CoSn7 units are condensed via common rectangular faces to [Co2Sn10] double units which build up a covalently bonded three-dimensional network through Sn-Co-Sn bridges. Larger voids left by this network are filled by the barium and the Sn2 atoms. The latter have distorted octahedral tin coordination with Sn2- Sn distances of 311 - 315 pm. The barium atoms have 13 nearest tin neighbors (352 - 399 pm Ba-Sn). Temperature-dependent magnetic susceptibility data of BaCo2Sn8 show Pauli paramagnetism.