Unexpected Discovery of a Binary Monoclinic Li7Sn2 Phase in the Li-Sn System

A new stoichiometric lithium-rich stannide, Li7Sn2, has been identified in the binary Li-Sn system. Single-crystal X-ray diffraction studies show that Li7Sn2 crystallizes in the monoclinic space group P21/c (Pearson index mP72) with a unique structure. For decades, an orthorhombic Li7Sn2 phase (Pearson index oC36) was known, although recent reinvestigation confirmed that the composition of the latter is Li13Sn4, or rather Li7-xSn2, where x ≈ 1/2. The experimental discovery of the monoclinic Li7Sn2 phase, which is not just a crystallographically ordered derivative within a reported structural family, brings new important knowledge with regard to the Li-Sn phase diagram. Specifically, density functional theory calculations show negligible energy differences of 1-2 meV/atom between the monoclinic Li7Sn2 and the orthorhombic Li7-xSn2 phases. This near-isoenergetic landscape in a narrow compositional window (3.25 < x < 3.5 in LixSn) supports the coexistence of multiple energetically competitive phases, which is consistent with the strong pathway and temperature dependence of phase selection observed experimentally. Electronic band structure calculations indicate metallic behavior with a deep pseudogap at the Fermi level. The presented experimental and theoretical results demonstrate that subtle compositional changes in the lithium-rich region of the Li-Sn system generate distinct structures and differentiated bonding patterns.