Synthesis, structure, and bonding of hypoelectronic SrIn₄: Direct example of a dominant size effect in structure selection

The structure of the new title compound, the same type as EuIn₄, was refined in the monoclinic space group C2/m, with Z = 4. The compound exhibits a complex three-dimensional network built of four-and five-bonded indium atoms in fused and interbonded pentagons that sandwich the strontium atoms. Both electronic band-structure calculations and property measurements show that the compound is metallic. A detailed band-structure analysis indicates that the compound is hypoelectronic with a one-electron deficiency, but the In - In bonding is effectively optimized in the structure. The important role of cation size in the structure choice is noted in a comparative study of BaIn₄, which has the closely related BaAl₄-type structure in which barium atoms are sandwiched by six-membered rings.