Defect-assisted conductivity in organic ionic plastic crystals.

In order to determine the role of defects (vacancies and extended lattice defects) in the conductivity mechanism of a well studied organic ionic plastic crystal electrolyte, conductivity and mean defect volumes were measured. The ionic conductivity of the salt showed a characteristic phase dependence. Defect volumes, as measured by positron annihilation lifetime spectroscopy, showed increasing rates of expansion with increasing rotational disorder. The dependence of ionic conductivity on defect volume was observed to be phase dependent. Increases in mean defect volume size below approximately 100 cm(3) mol(-1) did not always facilitate ionic conductivity. It was shown that the material undergoes a solid-solid phase transition to the most disordered phase (a plastic crystalline phase with the highest conductivity) when the mean defect volume becomes larger than the molar volume of either the rotating anionic or cationic species. Conductivity in this phase had the strongest dependence on defect volume. Critical volumes calculated from the free volume model of Cohen and Turnbull were unrealistically large.