Impurity effects in the organic charge transfer salt Qn (TCNQ)2 [antikvár]

Impurities and lattice defects play a significant role in various properties of organic charge transfer salts due mainly to the low dimensional character of these materials. Lattice irregularities usually decrease the conductivity and smear out the phase transitions observed in TTF-TCNQ and related salts (Heeger), while in materials with low conductivity impurity conduction may be important. In another class of materials of which NMePh-TCUQ is an example, the varying disorder was claimed to be responsible for variations in the magnetic susceptibility (Theodoru), differences in the conductivity, on the other hand were suggested to be due to impurity effects (Coleman et al 1973). Quinolinium(TCNQ)g belongs to this latter class of organic conductors, both the magnetic and the electric properties of this salt are similar to NMPh-TCNQ. We have observed large variations of the conductivity depending on crystal preparation technique, and correlate the observed conductivity with variations of the amount of doubly ionized TCNQ detected by optical measurements in solution. Qn(TCNQ)2 was prepared in different ways, including also crystal growing techniques. Pig. 1 shows lg 6 versus 1/T curves of samples prepared in different ways, the high temperature part of the conductivity is shown in Pig. 2. Sample A was prepared by reacting quinolinium hydro-iodide with TCMQ in acetonitrile solution. Pine needles (dimensions 3x0,02x0.02 mm^) precipitated after a few hours. We have observed the same conductivity for crystals prepared from TCNQ and quinoline using durohydroquinone as a proton-electron source (Melby et al 1962).