Quaternary Zintl Phases Ba₂InSnP₃ and Ba₂InSnAs₃ with Complex Structures and n-Type Semiconducting Behavior

Presented are the crystal structures and the transport properties of the novel quaternary pnictides with the general formula Ba₃InSnPn₃ (Pn = P, As). Large single crystals of the phosphide, arsenide, and the P/As solid solution can be readily synthesized by using reactions with molten Sn as a choice of a reactive metal flux. The title compounds crystallize in a monoclinic crystal system with a new structure type. The structure features a 3D polyanionic sublattice of corner- and edge-sharing [M₄Pn₁₀] units made of double-corner-sharing ethane-like [M₂Pn₆] fragments (M = In/Sn). Comprehensive crystallographic studies, including single-crystal neutron diffraction, confirm the random distribution of In and Sn atoms; i.e., there is no long-range ordering. The equiatomic statistical distribution of In and Sn allows for the simultaneous existence of In-In, Sn-Sn, as well as In-Sn bonds, yielding closed-shell electronic configuration in atypical In²⁺ and Sn³⁺ states. Partitioning of the valence electrons as (Ba²⁺)₃(In²⁺)(Sn³⁺)(Pn^3-)₃ shows charge-balanced compositions; i.e., the compounds are Zintl phases. The expected valence-precise behavior is corroborated by both electronic structure calculations and electrical resistivity measurements, which show intrinsic narrow band gap semiconductors. Thermal and electrical transport measurements for single-crystalline Ba₃InSnAs₃ indicate an n-type charge transport mechanism with a relatively low charge-carrier concentration and mobility of electrons as the dominant charge carriers (n_300K ≈ 1 × 10¹⁷ cm^-3, μ_300K ≈ 4 cm²/V s).