Long-range carrier-mediated Cu-Cu interactions and low-temperature transitions in the quasi-one-dimensional CuxNi1-x(phthalocyanine)I alloys

A series of alloys CuxNi1-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 molecular orbitals of the PC macrocycles. The Cu2+ (S=(1/2) local moments of CuxNi1-x (PC)I remain exchange coupled even when the paramagnetic metal-ion chain incorporated within the M(PC) stacks is diluted (x1) with the diamagnetic Ni2+ ions and the Cu magnetization is also coupled to the itinerant -electron charge carriers. For alloys with x0.1, 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 Ta25 K, roughly independent of composition for 0.05x<1. A more dramatic response of the linewidth occurs upon cooling through Tb, 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 T2.3 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 (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.