Pressure-induced ionic to mixed ionic and electronic conduction transition in solid electrolyte LaF3

Abstract

The ionic transport properties of solid electrolyte LaF₃ were systematically studied under high pressures up to 30.6 GPa with alternate-current impedance spectra measurements and first-principles calculations. From the impedance spectra measurements, LaF₃ was found to transform from pure ionic conduction to mixed ionic and electronic conduction at 15.0 GPa, which results from the pressure-induced structural phase transition from a tysonite-type structure to an anti-Cu₃Ti-type structure. F⁻ ion migration can be suppressed by pressure, causing a decrease of the ionic conductivity of LaF₃. By first-principles calculations, the pressure-dependent diffusion behaviors of the F⁻ ions can be understood. The increased overlap of electron clouds at the interstitial site between rigid La³⁺ and liquid F⁻ lattices leads to the appearance of electronic conduction in anti-Cu₃Ti-type structured LaF₃.