ChemInform Abstract: Dielectric Study on Ionic Orientational Disorder in the Low-Temperature Phases of Ionic Plastic Crystal KNO2.

The complex dielectric permittivity of ionic plastic crystal KNO2 was measured in the frequency range 20-10(6) Hz and the temperature range 22-300 K. A small step-like anomaly of the static dielectric permittivity appeared at the rhombohedral (phase II) to monoclinic (phase III) transition (T-tr2 = 264.1 K). In phase III, the static dielectric permittivity decreased gradually on cooling over a wide temperature range 50-264 K. These results indicate that the orientation of the NO2- ion is still disordered at high temperatures in phase III and becomes ordered gradually with decreasing temperature. The dielectric dispersion associated with this motion occurred in-the temperature range 70-150 K. From these data and the previously reported N-15 NMR data, we propose a model of the motion of NO2- in phase III in which the anion undergoes 180 degrees-flip motion about the axes perpendicular to the molecular C-2 axis in an asymmetric double-minimum potential.