title: Pressure induced phase transitions and metallization of a neutral radical conductor.
abstract: The crystal structure and charge transport properties of the prototypal oxobenzene-bridged 1,2,3-bisdithiazolyl radical conductor 3a are strongly dependent on pressure. Compression of the as-crystallized α-phase, space group Fdd2, to 3-4 GPa leads to its conversion into a second or β-phase, in which F-centering is lost. The space group symmetry is lowered to Pbn2₁, and there is concomitant halving of the a and b axes. A third or γ-phase, also space group Pbn2₁, is generated by further compression to 8 GPa. The changes in packing that accompany both phase transitions are associated with an "ironing out" of the ruffled ribbon-like architecture of the α-phase, so that consecutive radicals along the ribbons are rendered more nearly coplanar. In the β-phase the planar ribbons are propagated along the b-glides, while in the γ-phase they follow the n-glides. At ambient pressure 3a is a Mott insulator, displaying high but activated conductivity, with σ(300 K) = 6 × 10(-3) S cm(-1) and E(act) = 0.16 eV. With compression beyond 4 GPa, its conductivity is increased by 3 orders of magnitude, and the thermal activation energy is reduced to zero, heralding the formation of a metallic state. High pressure infrared absorption and reflectivity measurements are consistent with closure of the Mott-Hubbard gap near 4-5 GPa. The results are discussed in the light of DFT calculations on the molecular and band electronic structure of 3a. The presence of a low-lying LUMO in 3a gives rise to high electron affinity which, in turn, creates an electronically much softer radical with a low onsite Coulomb potential U. In addition, considerable crystal orbital (SOMO/LUMO) mixing occurs upon pressurization, so that a metallic state is readily achieved at relatively low applied pressure.
authors.affiliation: Department of Chemistry, University of Waterloo , Waterloo, Ontario N2L 3G1, Canada.
authors.forename: Joanne W L
authors.initials: JW
authors.lastname: Wong
authors.forename: Aaron
authors.initials: A
authors.lastname: Mailman
authors.forename: Kristina
authors.initials: K
authors.lastname: Lekin
authors.forename: Stephen M
authors.initials: SM
authors.lastname: Winter
authors.forename: Wenjun
authors.initials: W
authors.lastname: Yong
authors.forename: Jianbao
authors.initials: J
authors.lastname: Zhao
authors.forename: Subrahmanyam V
authors.initials: SV
authors.lastname: Garimella
authors.forename: John S
authors.initials: JS
authors.lastname: Tse
authors.forename: Richard A
authors.initials: RA
authors.lastname: Secco
authors.forename: Serge
authors.initials: S
authors.lastname: Desgreniers
authors.forename: Yasuo
authors.initials: Y
authors.lastname: Ohishi
authors.forename: Ferenc
authors.initials: F
authors.lastname: Borondics
authors.forename: Richard T
authors.initials: RT
authors.lastname: Oakley