Na+ diffusivity in carbon-substituted nido- and closo-hydroborate salts: Pulsed-field-gradient NMR studies of Na-7-CB10H13 and Na2(CB9H10)(CB11H12)

Abstract

The mixed-anion solid-solution closo-carbahydroborate Na2(CB9H10)(CB11H12) shows the highest room-temperature ionic conductivity among all known solid Na-ion and Li-ion conductors, and the related nido-type carbahydroborate Na-7-CB10H13 exhibits superionic conductivity above the order-disorder phase transition temperature, ∼320 K. To study the Na-ion diffusivity that is closely related to the ionic conductivity in these compounds, we have measured the diffusion coefficients of Na+ cations in Na2(CB9H10)(CB11H12) and Na-7-CB10H13 using the pulsed-field-gradient (PFG) spin-echo technique over the temperature ranges of 298–403 K and 320–403 K, respectively. These measurements have revealed the exceptionally high Na+ diffusivities (exceeding 2 × 10−6 cm2/s) for both compounds. In the studied temperature ranges, the diffusion coefficients are found to follow the Arrhenius law with the activation energies of 118(1) meV for Na2(CB9H10)(CB11H12) and 134(3) meV for Na-7-CB10H13. For the nido-type Na-7-CB10H13, the diffusivity results are complemented by the 1H and 23Na NMR and quasielastic neutron scattering measurements of the atomic jump rates. It is found that the transition from the low-T ordered phase to the high-T disordered phase occurring near 320 K is accompanied by the abrupt acceleration of both the reorientational jump rate of the [CB10H13]− anions and the diffusive jump rate of Na+ cations. For both compounds, a comparison of the measured Na+ diffusion coefficients with the ionic conductivity results and the data on the cation and anion jump rates provides new insights into unusual dynamical properties of these superionic materials.