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

In this paper, 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 ΔS_mag ≈ 1.6 J K^–1 (mol-Mn)^-1 across the antiferromagnetic transition. Finally, 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.