Anion Reorientations in the Superionic Conducting Phase of Na2B12H12

Quasielastic neutron scattering (QENS) methods were used to characterize the reorientational dynamics of the dodecahydro-closo-dodecaborate (B12H122–) anions in the high-temperature, superionic conducting phase of Na2B12H12. The icosahedral anions in this disordered cubic phase were found to undergo rapid reorientational motions, on the order of 1011 jumps s–1 above 530 K, consistent with previous NMR measurements and neutron elastic-scattering fixed-window scans. QENS measurements as a function of the neutron momentum transfer suggest a reorientational mechanism dominated by small-angle jumps around a single axis. The results show a relatively low activation energy for reorientation of 259 meV (25 kJ mol–1).