Intricate Li–Sn Disorder in Rare-Earth Metal–Lithium Stannides. Crystal Chemistry of RE₃Li_4–xSn_4+x (RE = La–Nd, Sm; x < 0.3) and Eu₇Li_8–xSn_10+x (x ≈ 2.0)

Reported are the syntheses, crystal structures, and electronic structures of six rare-earth metal–lithium stannides with the general formulas RE₃Li_4–xSn_4+x (RE = La–Nd, Sm) and Eu₇Li_8–xSn_10+x. These new ternary compounds have been synthesized by high-temperature reactions of the corresponding elements. Their crystal structures have been established using single-crystal X-ray diffraction methods. The RE₃Li_4–xSn_4+x phases crystallize in the orthorhombic body-centered space group Immm (No. 71) with the Zr₃Cu₄Si₄ structure type (Pearson code oI22), and the Eu₇Li_8–xSn_10+x phase crystallizes in the orthorhombic base-centered space group Cmmm (No. 65) with the Ce₇Li₈Ge₁₀ structure type (Pearson code oC50). Both structures can be consdered as part of the [RESn₂]_n[RELi₂Sn]_m homologous series, wherein the structures are intergrowths of imaginary RESn₂ (AlB₂-like structure type) and RELi₂Sn (MgAl₂Cu-like structure type) fragments. Close examination the structures indicates complex occupational Li–Sn disorder, apparently governed by the drive of the structure to achieve an optimal number of valence electrons. This conclusion based on experimental results is supported by detailed electronic structure calculations, carried out using the tight-binding linear muffin-tin orbital method.