Unique [Mn₆Bi₅]⁻ Nanowires in KMn₆Bi₅: A Quasi-One-Dimensional Antiferromagnetic Metal

Abstract

We report a new quasi-one-dimensional compound KMn₆Bi₅ composed of parallel nanowires crystallizing in a monoclinic space group C2/ m with a = 22.994(2) Å, b = 4.6128(3) Å, c = 13.3830(13) Å and β = 124.578(6)°. The nanowires are infinite [Mn₆Bi₅]^- columns each of which is composed of a nanotube of Bi atoms acting as the cladding with a nanorod of Mn atoms located in the central axis of the nanotubes. The nanorods of Mn atoms inside the Bi cladding are stabilized by Mn-Mn bonding and are defined by distorted Mn-centered cluster icosahedra of Mn₁₃ sharing their vertices along the b axis. The [Mn₆Bi₅]^- nanowires are linked with weak internanowire Bi-Bi bonds and charge balanced with K⁺ ions. The [Mn₆Bi₅]^- nanowires were directly imaged by high-resolution transmission electron microscopy and scanning transmission electron microscopy. Magnetic susceptibility studies show one-dimensional characteristics with an antiferromagnetic transition at ∼75 K and a small average effective magnetic moment (1.56 μ_B/Mn for H ∥ b and 1.37 μ_B/Mn for H ⊥ b) of Mn from Curie-Weiss fits above 150 K. Specific heat measurements reveal an electronic specific heat coefficient γ of 6.5(2) mJ K^-2(mol-Mn)^-1 and a small magnetic entropy change Δ S_mag ≈ 1.6 J K^-1 (mol-Mn)^-1 across the antiferromagnetic transition. In contrast to a metallic resistivity along the column, the resistivity perpendicular to the column shows a change from a semiconducting behavior at high temperatures to a metallic one at low temperatures, indicating an incoherent-to-coherent crossover of the intercolumn tunneling of electrons.