Interstitialcy diffusion of fluoride ions in LaOF by DFT-based first-principles calculations

Abstract

Ionic conduction in LaOF was investigated by density functional theory-based first-principles calculations. The origin and microscopic mechanism of selective fluoride ion conduction in LaOF were investigated by evaluating the formation energy of the Frenkel pair and by performing ab initio molecular dynamics (MD) simulations for both tetragonal (anion order along the [0 0 1] direction) and rhombohedral (anion order along the [1 1 1] direction) structures experimentally obtained in LaO1−xF1+2x. In both structures, the formation of an F Frenkel pair was energetically favored over that of an O Frenkel pair, and the F Frenkel pair predominantly contributed to the conduction of fluoride ions. From the trajectory of MD simulations with an F or O Frenkel pair in the two structures, it was found that fluoride ion conduction mainly occurred via an interstitialcy mechanism. This fluoride ion migration occurred only in the tetragonal structure along the (0 0 1) plane. This observation was further confirmed by migration barriers evaluated by the climbing-image nudged elastic band method. The results suggested the possibility of the enhancement of ionic conductivity by controlling the anion ordering in mixed anion compounds.