Soloninin A, Skoryunov R, Babanova O, Skripov A, Dimitrievska M, Udovic T. Comparison of anion and cation dynamics in a carbon-substituted
closo-hydroborate salt:
1H and
23Na NMR studies of solid-solution Na
2(CB
9H
10)(CB
11H
12).
JOURNAL OF ALLOYS AND COMPOUNDS 2019;
800:10.1016/j.jallcom.2019.06.019. [PMID:
38903956 PMCID:
PMC11187972 DOI:
10.1016/j.jallcom.2019.06.019]
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Abstract
The hexagonal mixed-anion solid solution Na2(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 1H and 23Na nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates in Na2(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 Na2(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 1H spin-lattice relaxation rates for Na2(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.
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