X-ray-scattering study of the metal-phthalocyanine-iodine quasi-one-dimensional molecular conductors

X-ray-scattering measurements on several newly synthesized, high-purity metal-phthalocyanine-iodine quasi-one-dimensional conductors reveal that, in contrast to previous materials, the iodine sublattice is ordered at all temperatures. The crystal symmetry is thus modified from tetragonal P4/mcc to monoclinic P2/c with a supercell unit volume three times larger. This new symmetry, which does not modify the dispersion of the one-dimensional band structure in the vicinity of the Fermi level, is consistent with the metallic behavior of the M(pc)I family. However, a metal-insulator transition could be induced if the c glide-plane symmetry of P2/c is broken due to the stabilization of a 2kF density-wave ground state.