Comparison of anion and cation dynamics in a carbon-substituted -hydroborate salt: H and Na NMR studies of solid-solution Na(CBH)(CBH).

The hexagonal mixed-anion solid solution Na(CBH)(CBH) 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 and Na nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates in Na(CBH)(CBH) 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(CBH)(CBH) 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 spin-lattice relaxation rates for Na(CBH)(CBH) 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.