Superionic Character of High-Temperature Conduction in Lanthanum Fluoride

The electroconduction of crystalline lanthanum fluoride is studied in the temperature range from 1320 K to the melting point (1766 K). A markedly pronounced superionic behavior of LaF₃ is established: near mp its conductivity amounts to 42% of that corresponding to a molten state. Thermodynamics of the formation of defects and their concentrations are estimated on the basis of available data on the high-temperature enthalpy of LaF₃ within a model of noninteracting defects. The Schottky and Frenkel mechanisms of ion disordering are considered. A comparative analysis of these two mechanisms supplemented with results on the conductivity favors the Frenkel mechanism of the origin of defects in LaF₃.