Modified Anion Packing of Na₂B₁₂H₁₂ in Close to Room Temperature Superionic Conductors

Three different types of anion packing, i.e., hexagonal close packed (hcp), cubic close packed (ccp), and body centered cubic (bcc), are investigated experimentally and with ab initio calculations in the model system Na₂B₁₂H₁₂. Solvent free and water assisted mechanical grinding provide polycrystalline samples for temperature-dependent synchrotron radiation X-ray powder diffraction and electrochemical impedance spectroscopy. It is shown that among the common close packed lattices, the hcp anionic backbone creates very favorable conditions for three-dimensional ionic conduction pathways, comprised of O-O, T-T, and T-O-T (O for octahedral, T for tetrahedral) cation hops. The hcp lattice is stable with respect to ccp and bcc lattices only at higher volumes per formula unit, which is achieved either by cationic substitution with larger cations or partial substitution of hydrogen by iodine on the closo-anion. It is found that the partial cationic substitution of sodium with lithium, potassium, or cesium does not lead to enhanced conductivity due to the obstruction of the conduction pathway by the larger cation located on the octahedral site. Substitution on the closo-anion itself shows remarkable positive effects, the ionic conductivity of Na₂B₁₂H_12-xI_x reaching values of close to 10^-1 S cm^-1 at a rather low temperature of 360 K. While the absolute value of σ is comparable to that of NaCB₁₁H₁₂, the temperature at which it is attained is approximately 20 K lower. The activation energy of 140 meV is determined from the Arrhenius relation and among the lowest ever reported for a Na-conducting solid.