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

Abstract

The temperature dependence (in the range of 479–827 K) of the fluorine-ionic conductivity of nonstoichiometric Na_0.37Tb_0.63F_2.26 crystals with fluorite-type structure (sp. gr. \(Fm\bar {3}m\), lattice parameter a = 5.5785(1) Å) 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 Na_0.37Tb_0.63F_2.26 crystals. The ionic conductivity is found to be σ_dc = 2.6 × 10^–6 and 3 × 10^–3 S/cm at 500 and 827 K, respectively. A crystallophysical model of ion transport in Na_0.37Tb_0.63F_2.26 crystals is proposed. The concentration and carrier mobility are calculated to be, respectively, n_mob = 1.45 × 10²¹ cm^–3 and μ_mob = 1.1 × 10^–8 cm²/(s V) at 500 K.