Conductivity and Polarization Phenomena in Phenanthrene Crystals

Dark conductivity and photoconductivity perpendicular to the a—b plane in phenanthrene single crystals were investigated. The dark conductivity shows anomalous behavior in the region of 72°C, the same temperature at which an anomaly in the heat capacity appears. An Arrhenius plot of equilibrium resistivity yields an activation energy of 1.5 eV below 72°C and 1.1 eV above 72°C. When a crystal is cooled from above 72°C in an electric field, a polarization charge is detected which is shown to be a volume rather than a surface phenomenon. When a crystal is illuminated and heated above 72°C in an electric field, it becomes permanently polarized. Electrical-conductivity measurements show that there can be photo-carrier motion in addition to the polarization current. No anomalous behavior was found in x-ray, thermal-expansion, or dielectric measurements.