Physical properties of electrically conducting and stable molecular neutral radical solid [Co(2,3-Nc)(CN)2]CH3CN (2,3-Nc = 2,3-naphthalocyanine)

Abstract

Solid state properties of dicyano(2,3-naphthalocyaninato)cobalt(III) neutral radical crystal, [ Co (2,3- Nc )( CN )2] CH 3 CN , were characterized by measurements of the resistivity under high pressure and under uniaxial strain, thermoelectric power, magnetic susceptibility, ESR and polarized reflectance spectra. The title compound exhibited thermally activated-type electrical conductivity along the c-axis. The room temperature (RT) resistivity ρRT along the c-axis and activation energy Ea rapidly decreased with increasing pressure. The temperature-dependent thermoelectric power S was that of a typical one-dimensional (1D) semiconductor. However the high absolute value of S suggested that this electronic system should be strongly correlated. Although the electrical resistivity exhibited monotonous temperature-dependence, the magnetic susceptibility clearly indicated a Peierls-type transition and marked fluctuation from RT. Both Peierls-type transitions and fluctuations are characteristic phenomena to 1D conductors. Furthermore ESR spectra manifested that the Peierls-type transition occurred at 100 K. The inconsistency between the electrical behavior (without a phase-transition) and magnetic behavior (with a phase-transition) indicates separation of the degrees of freedom in spin and charge (spin-charge separation) of this material. Spin-charge separation is a theoretically predicted phenomenon peculiar to the 1D conductors with strong correlation. The reflectance spectra were quantitatively explained by a 1D Hubbard model, and manifested the existence of a structural fluctuation of this material from RT. Based on these observed physical properties it is concluded that [ Co (2,3- Nc )( CN )2] CH 3 CN is a strongly correlated 1D semiconductor with a Mott(Hubbard) type energy gap and characterised with a fluctuation and spin-charge separation.