Dielectric study on ionic orientational disorder in the low-temperature phases of ionic plastic crystal KNO2

Article abstract of DOI:10.1002/bbpc.19981020204

The complex dielectric permittivity of ionic plastic crystal KNO₂ was measured in the frequency range 20-10⁶ 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 HI 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 ¹⁵N NMR data, we propose a model of the motion of NO₂^- in phase III in which the anion undergoes 180°-flip motion about the axes perpendicular to the molecular C₂ axis in an asymmetric double-minimum potential. WILEY-VCH Verlag GmbH, 1998.