The Metallic State in Neutral Radical Conductors: Dimensionality, Pressure and Multiple Orbital Effects

Pressure-induced
changes in the solid-state structures and transport
properties of three oxobenzene-bridged bisdithiazolyl radicals 2 (R = H, F, Ph) over the range 0–15 GPa are described.
All three materials experience compression of their π-stacked
architecture, be it (i) 1D ABABAB π-stack (R = Ph), (ii) quasi-1D
slipped π-stack (R = H), or (iii) 2D brick-wall π-stack
(R = F). While R = H undergoes two structural phase transitions, neither
of R = F, Ph display any phase change. All three radicals order as
spin-canted antiferromagnets, but spin-canted ordering is lost at
pressures <1.5 GPa. At room temperature, their electrical conductivity
increases rapidly with pressure, and the thermal activation energy
for conduction Eact is eliminated at pressures
ranging from ∼3 GPa for R = F to ∼12 GPa for R = Ph,
heralding formation of a highly correlated (or bad) metallic state.
For R = F, H the pressure-induced Mott insulator to metal conversion
has been tracked by measurements of optical conductivity at ambient
temperature and electrical resistivity at low temperature. For R =
F compression to 6.2 GPa leads to a quasiquadratic temperature dependence
of the resistivity over the range 5–300 K, consistent with
formation of a 2D Fermi liquid state. DFT band structure calculations
suggest that the ease of metallization of these radicals can be ascribed
to their multiorbital character. Mixing and overlap of SOMO- and LUMO-based
bands affords an increased kinetic energy stabilization of the metallic
state relative to a single SOMO-based band system.