Long-range carrier-mediated Cu-Cu interactions and low-temperature transitions in the quasi-one-dimensional Cu_xNi_1-x(phthalocyanine)I alloys

A series of alloys Cu_xNi_1-x(PC)I (PC = phthalocyanine) of the two isostructural molecular conductors, phthalocyaninato nickel(II) iodide, Ni(PC)I, and phthalocyaninato copper(II) iodide, Cu(PC)I, have been prepared. These crystals contain partially oxidized M(PC) stacks and are quasi-one-dimensional molecular metals whose charge carriers are associated with the highest occupied π molecular orbitals of the PC macrocycles. The Cu²⁺ (S=(1/2) local moments of Cu_xNi_1-x (PC)I remain exchange coupled even when the paramagnetic metal-ion chain incorporated within the M(PC) stacks is diluted (x≪1) with the diamagnetic Ni²⁺ ions and the Cu magnetization is also coupled to the itinerant π-electron charge carriers. For alloys with x≥0.1, the EPR signal of the coupled magnetization exhibits two anomalies at low temperature. The g values and linewidths first begin to deviate from their high-temperature behavior at T_a∼25 K, roughly independent of composition for 0.05≤x<1. A more dramatic response of the linewidth occurs upon cooling through T_b, which decreases from ∼8 K as x is reduced from 1.0. Surprisingly, the g value of the x=0.50 alloy at low temperature shows a field dependence: At X-band frequency, g_? increases to ∼2.21 by T∼2.3 K, a g value much larger than that of the parent Cu(PC) (g_?=2.18); this anomaly is quenched at a higher observing field (Q-band frequency). These alloys are highly conducting, as are the two parent materials. The dependence of the conductivity on x indicates that σ(T) is governed by magnetic scattering by the Cu²⁺ ions. In the low-temperature region, the results for the four-probe and microwave conductivity differ sharply in a composition-dependent fashion and indicate a novel coupling between dielectric and magnetic properties.