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 ($T\to 0$) value that varies from ½ to 2 times the value at room temperature (∼500 ${\mathrm{\Omega }}^{-1\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$). 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.