Plastic domain and ionic conductivity of (C4H9)4NI

Unusual ionic transport in tetrabutylammonium iodide (C4H9)4NI (TBAI) was observed by high resolution proton NMR spectroscopy accompanied by diffusion experiments. Some parts of the TBA cations are diffusing not only in the plastic phase II, but in the low temperature ordered phase I as well. The diffusing TBA ions are considered to be inside the remaining plastic domains dispersed in the ordered crystal matrix. The volume fraction of the plastic domain is evaluated from the observed narrow component of the proton NMR spectra. The observed unusual temperature dependence of the diffusion coefficient can be explained by assuming a manifold decrease of the dimension of the plastic domain in phase I. The differences in degree of crystallinity at the interface of plastic domains with respect to its surroundings led to the surface enhancement of diffusivity in phase I. Finally, the observed ionic conductivity is compared with the estimation from the diffusion coefficient and mobile cation densities. The results suggest that only a fraction of the cations take part in the observed ionic conductivity.