Unusual band-filling and counterion ordering effects in a phthalocyanine molecular metal. Single crystal studies of Ni(Pc) (C10₄)_y

The phthalocyanine molecular metal Ni(Pc) (C10₄)_y, y=0.39-0.47 for most crystals, has been synthesized by electrocrystallization techniques. The structure of a crystal with y=0.42 consists of staggered (39.5°) Ni(Pc)^+0.42 units stacked at 3.233(1) Å separations and disordered, off-axis C10^-₄ ions. At lower temperatures, the C10^-₄ ions undergo a transition to an ordered arrangement. At 300°K, σ_{{norm of matrix}}=700±200Ω^-1 cm^-1 with metal-like (dσ/dT<0) behavior as the temperature is lowered. A broad maximum in σ(T) is found near 200°K, with discontinuous conductivity behavior observed in all samples at lower temperatures. The cause appears to be domain wall phenomena associated with C10₄ ordering rather than macroscopic fracturing. Thermoelectric power data are also metal-like (S≈T) with no evidence of the aforementioned discontinuities. Room temperature values of S=20-35 μVK^-1 are in accord with the relatively high degree of Ni(Pc)^+ρ partial oxidation. Polarized single crystal reflectance data reveal a plasma edge, the high energy of which is also explicable in terms of the relatively high degree of partial oxidation. Analysis of the thermoelectric power and optical data yields, in both cases, a tight-binding bandwidth of ca. 1.5 eV.