Comparison of Anion Reorientational Dynamics in MCB₉H₁₀ and M₂B₁₀H₁₀ (M = Li, Na) via Nuclear Magnetic Resonance and Quasielastic Neutron Scattering Studies

The disordered phases of the 1-carba-closo-decaborates LiCB₉H₁₀ and NaCB₉H₁₀ exhibit the best solid-state ionic conductivities to date among all known polycrystalline competitors, likely facilitated in part by the highly orientationally mobile CB₉H₁₀^– anions. We have undertaken both NMR and quasielastic neutron scattering (QENS) measurements to help characterize the monovalent anion reorientational mobilities and mechanisms associated with these two compounds and to compare their anion reorientational behaviors with those for the divalent B₁₀H₁₀^2– anions in the related Li₂B₁₀H₁₀ and Na₂B₁₀H₁₀ compounds. NMR data show that the transition from the low-T ordered to the high-T disordered phase for both LiCB₉H₁₀ and NaCB₉H₁₀ is accompanied by a nearly two-orders-of-magnitude increase in the reorientational jump rate of CB₉H₁₀^– anions. QENS measurements of the various disordered compounds indicate anion jump correlation frequencies on the order of 10¹⁰–10¹¹ s^–1 and confirm that NaCB₉H₁₀ displays jump frequencies about 60% to 120% higher than those for LiCB₉H₁₀ and Na₂B₁₀H₁₀ at comparable temperatures. The Q-dependent quasielastic scattering suggests similar small-angular-jump reorientational mechanisms for the different disordered anions, changing from more uniaxial in character at lower temperatures to more multidimensional at higher temperatures, although still falling short of full three-dimensional rotational diffusion below 500 K within the nanosecond neutron window.