Synthesis, Structural Characterization and Chemical Bonding of Sr₇Li₆Sn₁₂ and its Quaternary Derivatives with Eu and Alkaline Earth Metal (Mg, Ca, Ba) Substitutions. A Tale of Seven Li-Containing Stannides and Two Complex Crystal Structures

In this paper, we discuss the synthesis and the structural characterization of the new ternary compound Sr7Li6Sn12 and its six quaternary derivatives, where alkaline earth metals (Ca, Mg, Ba) and the rare earth metal Eu are substituted, yielding crystalline phases of monoclinic or orthorhombic symmetry. The title compounds were synthesized via high-temperature solid-state reactions of the corresponding elements. The crystal structures were determined by single-crystal X-ray diffraction methods. The (Sr,Ca)(7)Li6Sn12 and Eu-7(Mg,Li)(6)Sn-12 phases represent a new structure type (space group P2/m, No. 10) while Sr7Li6Sn12 and the rest of the title phases crystallize in the orthorhombic base-centered space group Cmmm (No. 65) which is similar to the Eu7Li8-xSn10+x (x approximate to 2.0) phase with the Ce7Li8Ge10 structure type (Pearson code oC50). Careful examination of the resulting structures shows intricate disordering between Li and Sn atoms, which is governed by the total number of valence electrons. The discussion of experimental results is also supported by DFT electronic structure calculations.