title: Discovery of the K4 Structure Formed by a Triangular π Radical Anion.
abstract: The K4 structure was theoretically predicted for trivalent chemical species, such as sp(2) carbon. However, since attempts to synthesize the K4 carbon have not succeeded, this allotrope has been regarded as a crystal form that might not exist in nature. In the present work, we carried out electrochemical crystallization of the radical anion salts of a triangular molecule, naphthalene diimide (NDI)-Δ, using various electrolytes. X-ray crystal analysis of the obtained crystals revealed the K4 structure, which was formed by the unique intermolecular π overlap directed toward three directions from the triangular-shape NDI-Δ radical anions. Electron paramagnetic resonance and static magnetic measurements confirmed the radical anion state of NDI-Δ and indicated an antiferromagnetic intermolecular interaction with the Weiss constant of θ = -10 K. The band structure calculation suggested characteristic features of the present material, such as a metallic ground state, Dirac cones, and flat bands.
authors.forename: Asato
authors.initials: A
authors.lastname: Mizuno
authors.forename: Yoshiaki
authors.initials: Y
authors.lastname: Shuku
authors.affiliation: ∥Department of Nanomaterials Science, Graduate School of Advanced Integration Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan.
authors.forename: Rie
authors.initials: R
authors.lastname: Suizu
authors.forename: Michio M
authors.initials: MM
authors.lastname: Matsushita
authors.forename: Masahisa
authors.initials: M
authors.lastname: Tsuchiizu
authors.forename: Daniel
authors.initials: D
authors.lastname: Reta Mañeru
authors.forename: Francesc
authors.initials: F
authors.lastname: Illas
authors.affiliation: #Laboratorie de Chimie Quantique, Institut de Chimie-UMR 7177, Universitè de Strasbourg, 1 rue Blaise Pascal, 67000 Strasbourg, France.
authors.forename: Vincent
authors.initials: V
authors.lastname: Robert
authors.forename: Kunio
authors.initials: K
authors.lastname: Awaga