Dynamic Heterogeneity and Ionic Conduction in an Organic Ionic Plastic Crystal and the Role of Vacancies

Dynamic heterogeneity was investigated for the first time in a conductive organic ionic plastic crystal by molecular dynamics simulation. A minority fraction of ions that possess above-average dynamics were identified in the plastic crystal phase. The signature of this unusual motional behavior is found in the significant increase in the non-Gaussian parameter alpha(t). A study by incorporation of vacancies into the crystal structure shows explicit evidence of coexistence of mobile species with an otherwise rigid matrix, which particularly supports the previous explanation on heterogeneous motional narrowing in nuclear magnetic resonance. It is also found that the origin of dynamic heterogeneity here is inseparable from the inherent structural characteristics of organic ions. This work reveals the profound effect brought by heterogeneous dynamics on the conduction mechanism of this material, as well as the important role of defects on ions dynamics.