Fluorine-Ionic Conductivity of Superionic Conductor Crystals Na0.37Tb0.63F2.26

The temperature dependence (in the range of 479–827 K) of the fluorine-ionic conductivity of nonstoichiometric Na0.37Tb0.63F2.26 crystals with fluorite-type structure (sp. gr. $$Fm\bar {3}m$$, lattice parameter a = 5.5785(1) A) has been investigated. The crystals were grown from melt by the Bridgman method in a fluorinating atmosphere of CF4. The formation of cuboctahedral clusters of defects {(Na,Tb)8[Tb6F37]F32} is most likely in the structure of Na0.37Tb0.63F2.26 crystals. The ionic conductivity is found to be σdc = 2.6 × 10–6 and 3 × 10–3 S/cm at 500 and 827 K, respectively. A crystallophysical model of ion transport in Na0.37Tb0.63F2.26 crystals is proposed. The concentration and carrier mobility are calculated to be, respectively, nmob = 1.45 × 1021 cm–3 and μmob = 1.1 × 10–8 cm2/(s V) at 500 K.