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)7Li6Sn12 and Eu7(Mg,Li)6Sn12 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 ≈ 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. The orthorhombic Sr7Li6Sn12 crystal shows high degree of occupational disordering between Li and Sn atoms. This disorder is governed by the need for the structure to achieve optimum number of valence electrons. Upon partial substitution of Sr with Ca atoms, the symmetry distorts to monoclinic and the phase (Ca,Sr)7Li6Sn12 is realized. Therein, local distortion due to Ca and Sr atoms being mixed leads to ordering between Li and Sn atoms.