Comparison of anion and cation dynamics in a carbon-substituted closo-hydroborate salt: 1H and 23Na NMR studies of solid-solution Na2(CB9H10)(CB11H12)

The hexagonal mixed-anion solid solution Na-2(CB9H10)(CB11H12) shows the highest room-temperature ionic conductivity among all known Na-ion conductors. To study the dynamical properties of this compound, we have measured the H-1 and Na-23 nuclear magnetic resonance (NMR) spectra and spinlattice relaxation rates in Na-2(CB9H10)(CB11H12) over the temperature range of 80-435 K. It is found that the diffusive motion of Na+ ions can be described in terms of two jump processes: the fast localized motion within the pairs of tetrahedral interstitial sites of the hexagonal close-packed lattice formed by large anions and the slower jump process via octahedral sites leading to long-range diffusion. Below 350 K, the slower Na+ jump process is characterized by the activation energy of 353(11) meV. Although Na+ mobility in Na-2(CB9H10)(CB11H12) found from our NMR experiments is higher than in other ionic conductors, it appears to be an order-of-magnitude lower than that expected on the basis of the conductivity measurements. This result suggests that the complex diffusion mechanism and/or correlations between Na+ jumps should be taken into account. The measured H-1 spin-lattice relaxation rates for Na-2(CB9H10)(CB11H12) are consistent with a coexistence of at least two anion reorientational jump processes occurring at different frequency scales. Near room temperature, both reorientational processes are found to be faster than the Na+ jump process responsible for the long-range diffusion. (C) 2019 Elsevier B.V. All rights reserved.