LOW-DIMENSIONAL MOLECULAR CONDUCTORS (PER)2M(MNT)2, M = CU AND NI - LOW-CONDUCTIVITY AND HIGH-CONDUCTIVITY PHASES

Single crystals of the quasi-one-dimensional conductors (Per)2M(mnt)2 (Per = perylene, M = Ni and Cu, mnt = maleonitrile dithiolate or cis-2,3-dimercapto-2-butenedinitrile) were prepared by electrocrystallization. In addition to the previously reported beta-phase with semiconducting properties (sigma-RT congruent-to 50 OMEGA-1 cm-1, S(RT) = 22-mu-V K-1), a second phase with metallic properties, denoted alpha, was also obtained. The alpha- and beta-phases differ by their crystallographic structure. A temperature-independent superlattice modulation stabilizing the 6b periodicity in the chain direction characterizes the beta-phases. The crystal structure of the metallic alpha-phase consists of a close-packed arrangement of uniform stacks of segregated perylene and M(mnt)2 units. At room temperature, the electrical conductivity along the stacking axis is sigma-RT congruent-to 700 OMEGA-1 cm-1 and the absolute thermoelectric power has a positive value of S(RT) = 35-mu-V K-1. Both the Ni and Cu derivatives exhibit a metallic behavior down to 25 and 32 K, respectively, where a metal-insulator transition occurs.