Order–Disorder Transitions and Superionic Conductivity in the Sodium nido-Undeca(carba)borates

The salt compounds NaB 11 H 14, Na-7-CB 10 H 13, Li-7-CB 10 H 13, Na-7,8-C 2 B 9 H 12, and Na-7,9-C 2 B 9 H 12 all contain geometrically similar, monocharged, nido -undeca(carba)borate anions (i.e., truncated icosohedral-shaped clusters constructed of only 11 instead of 12 {B–H} + {C–H} vertices and an additional number of compensating bridging and/or terminal H atoms). We used first-principles calculations, X-ray powder diffraction, differential scanning calorimetry, neutron vibrational spectroscopy, neutron elastic-scattering fixed-window scans, quasielastic neutron scattering, and electrochemical impedance measurements to investigate their structures, bonding potentials, phase-transition behaviors, anion orientational mobilities, and ionic conductivities compared to those of their closo -poly(carba)borate cousins. All exhibited order–disorder phase transitions somewhere between room temperature and 375 K. All disordered phases appear to possess highly reorientationally mobile anions (> ∼10 10 jumps s –1 above 300 K) and cation-vacancy-rich, close-packed or body-center-cubic-packed structures [like previously investigated closo -poly(carba)borates]. Moreover, all disordered phases display superionic conductivities but with generally somewhat lower values compared to those for the related sodium and lithium salts with similar monocharged 1-CB 9 H 10 – and CB 11 H 12 – closo -carbaborate anions. This study significantly expands the known toolkit of solid-state, poly(carba)borate-based salts capable of superionic conductivities and provides valuable insights into the effect of crystal lattice, unit cell volume, number of carbon atoms incorporated into the anion, and charge polarization on ionic conductivity.