IONIC-CONDUCTIVITY IN ORDERED AND DISORDERED PHASES OF PLASTIC CRYSTALS

Pentaglycerine (PG) and tris-hydroxy-aminomethane (Tris) are potential thermal energy storage materials which undergo energetic solid-state phase transformations. The low temperature alpha-phase structure of PG is body-centered tetragonal and that of Tris is orthorhombic. The high-temperature phase structure of both compounds is face-centered cubic. Polymorphic changes in the structure occur at 89-degrees-C for PG and 135-degrees-C for Tris. These compounds are dielectrics at low temperature with conductivities on the order of 10(-8) OMEGA-1 cm-1(at 21-degrees-C) and 10(-9) OMEGA-1 cm-1 (at 25-degrees-C), for PG and Tris, respectively. Heating samples from room temperature leads to an initial decrease in a-phase conductivities followed by an increase for both of these compounds as a result of proton hopping. Continuous increases in the conductivity were observed at the transition temperature, with discontinuity evident in the case of PG. The conductivity variations in the alpha- and gamma-phases have been found to be thermally activated. The activation energies are nearly equal suggesting similar conduction mechanisms and charge carriers. From the relaxation time data, an equation has been obtained for the charge carrier self-diffusion coefficient for the gamma-phase of PG, just above the transition temperature; D(tau) = 0.085e-57,167/RT. The conductivity in the gamma-phases are independent of the frequency in the range of 10 to 10' Hz. The temperature and frequency effects on the conductivities, relaxation times, and diffusional parameters of PG and Tris are presented.