Compressibility and high-pressure electrical resistivity of one-dimensional phthalocyanine conductors: The relationship with d-pi charge transfer

Powder X-ray diffraction patterns of one-dimensional phthalocyanine conductors (phthalocyaninato)-nickel hexafluoroarsenate, NiPc(AsF6)(0.5) and (phthalocyaninato)-cobalt hexafluoroarsenate, CoPc(AsF6)(0.5), were measured under high pressure. The crystal symmetries of the both compounds were maintained in the pressure range of 0-3.5 GPa. In both compounds, the compressibility along the chain axis is larger than that along the interchain axis. The compressibility of NiPc(AsF6)(0.5) below 0.5 GPa is unusually larger than that of CoPc(AsF6)(0.5). The relation between the lattice contraction along the chain axis and the pressure-induced d-pi charge transfer (metal-ligand charge transfer) is discussed based on a simple ligand field theory. The enhancement of the electrical resistivity was observed at about 0.5 GPa in NiPc(AsF6)(0.5). This enhancement is consistent with the suggestion by the analysis of the pressure-dependent plasmon absorption. This resistivity change is attributed to the Anderson localization accompanying the metal-ligand charge transfer.