Nickel phthalocyanine iodide: A highly-one-dimensional low-temperature molecular metal

The conductivity of the linear-chain molecular-metal phthalocyanatonickel(II)iodide [Ni(pc)I] remains metallic down to {~}30 K, goes through a weak maximum, and then levels off to a high asymptotic (T→0) value that varies from {} to 2 times the value at room temperature ({~}500 Ω^-1 cm^-1). The full characterization of this compound, reported here, includes a thermoelectric power linearly proportional to T, a Pauli-like static and EPR magnetic susceptibility, and single-crystal reflectivity spectra exhibiting a plasma edge for light polarized parallel to the conducting axis. The results confirm that this compound possesses all the characteristics of molecular metal that retains a metallic band structure down to a temperature below 2 K. However, Ni(pc)I does not possess any strong interstack interactions of the type necessary to suppress a Peierls metal-nonmetal transition via an increase in the dimensionality, and should be classified as one of the most one-dimensional molecular metals studied to date. A structurally imposed weakening of the interstack Coulomb interactions, coupled with weak random potentials from structurally disordered triiodide chains, is apparently sufficient to suppress the three-dimensional transition normally associated with the tendency of an anisotropic conductor to undergo a Peierls distortion.