On the structure of heptalithium distannide: Li₇Sn₂ or Li_7-xSn₂ (0.3 < x < 0.5)

Li-Sn intermetallics are promising candidates for high-capacity Li-ion battery anodes, yet, the precise crystal structures of some "known" binary phases remain unresolved. One such phase, Li₇Sn₂, originally reported in 1975, is re-examined here using single-crystal X-ray diffraction methods. Its orthorhombic structure (space group Cmmm, no. 65) features isolated Sn atoms and Sn-Sn dumbbells in a matrix of Li atoms. The present study indicates occupational disorder on one of the Li sites in Li₇Sn₂, meaning that it is a non-stoichiometric phase with revised composition of Li_7-xSn₂ (0.3 < x < 0.5). In the most extreme case, when the Li atomic site in question is completely empty, the composition becomes Li_6.5Sn₂ or Li₁₃Sn₄. Analysis of the chemical bonding through electronic structure calculations indicates significant covalency of Sn-Sn and Li-Sn interactions.