The Semiconductive Property and the Phase Transition of the [(C6H5)3PCH3]+(TCNQ)2\ewdot Anion Radical Salt. The Entropy Change Due to Electrons and Holes

The semiconductive organic crystal of [(C6H5)3PCH3]+(TCNQ)2− is known to undergo a first-order phase transition at 315.7°K. The discontinuity in the temperature dependence of the electrical conductivity takes place at the transition temperature, above which the populations of the electrons and the holes in the electrical conductivity increase abruptly. A theoretical consideration of the system of conduction carriers in an intrinsic semiconductor was developed in order to evaluate the entropy change of the electrons and holes at the phase transition. For [(C6H5)3PCH3]+ (TCNQ)2−, this kind of entropy change was found to be much less than the total entropy change determined from the heat-capacity measurements by Kosaki et al. In view of these results, it was concluded that the anomaly in the electrical conductivity was induced by the change in the crystal structure.