engleski

Dynamic correlations in ionic conductor Cu2HgI4 probed by NMR

Solid state ionic conductivity is of immense technical importance, mainly because of battery technology. Yet it is not always clear what makes a substance undergo a transition from insulator to ionic conductor, and how the mobile ions manage to travel around the lattice relatively undisturbed. Here we discuss the ionic transport in Cu2HgI4, a well known fast ion conductor that undergoes an insulator-conductor transition at 344 K. NMR measurements provide ion-selective insight into dynamics, and we show that conduction is, surprisingly, mediated by the heavy Hg2+ ions, rather than much lighter and smaller Cu+. Although disordering of the Cu sublattice occurs at the transition, copper ions seem to remain static, while mercury motion is evident from a strong narrowing of the 199Hg NMR line. However, conductivity calculated from NMR linewidths shows a systematic discrepancy from DC conductivity, so frequency-dependent nonlinear conductivity measurements were used as a probe for dynamical heterogeneity. They reveal an 'intermediate' characteristic correlation timescale tau_corr ~1 ms. The existence of dynamical correlations is confirmed by stimulated echo NMR measurements, where the fraction of ions trapped after tau_corr can be quantitatively determined. To explain the observations, we propose a simple model requiring the existence of two kinds of particles, slow and fast. A simulation based on the model shows islands of trapped fast particles forming and dissolving in time, with a characteristic timescale similar to the one observed experimentally. The observed dynamical cooperativity can be compared with recent studies of arrested and glassy materials, thus opening an unexpected connection between seemingly unrelated fields of physics.

ionic conductors; NMR; dynamic correlations

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