Synthesis, structure, and bonding of open-shell Sr₃In₅: An unusual electron deficiency in an indium network, beyond the Zintl boundary

The new title compound has been synthesized and characterized by physical property measurements and electronic structure calculations. The results ratify the highly uncommon deficiency of one electron that has been long speculated for its Ca₃Ga₅-type structure on the basis of the simple Zintl electron counting formalism. In the Sr₃In₅ structure (Cmcm), 4- and 2-bonded indium atoms in a 4:1 ratio form a three-dimensional classical network that encapsulates strontium atoms in its narrow channels. The electrical conductivity of the compound shows typical metallic behavior. The detailed electronic structure analysis suggests that the electron hole is mainly localized on a nonbonding p-orbital on the 2-bonded indium atoms, and that these orbitals, stacked in a σ-type way along a (4.97 Å), interact only weakly with each other to form highly one-dimensional bands.