Haven Correlation Parameter for Diffusion of Fluorine in Superionic Conductors La1 – ySryF3 – y

Abstract

The processes of electric charge transfer (conductivity) and mass transfer (diffusion) in La1 ‒ ySryF3 – y superionic conductors are determined by mobile fluorine ions. The fluorine random-diffusion coefficients Dσ have been calculated from experimental data on the ionic conductivity σdc for La1 ‒ ySryF3 – y single crystals at the SrF2 dopant contents of 1, 3, 5, 7.5, 10, and 15 mol %. A maximum in the region of 3–5 mol % SrF2 is observed in the dependence Dσ(y). The value of the Haven correlation parameter Hr = DNMR/Dσ (DNMR is the fluorine diffusion coefficient measured by NMR on 19F nuclei) is determined; it characterizes the ion transport mechanism in La1 – ySryF3 – y crystals. The Hr values are 0.75 ± 0.15, 0.45 ± 0.15, and 0.65 ± 0.15 at 400–800 K for 1, 3, and 15–16 mol % SrF2, respectively. Superionic conductor La0.97Sr0.03F2.97 with maximum σdc and Dσ values has a minimum Haven parameter. The obtained Hr values indicate that fluorine diffusion in superionic conductors La1 – ySryF3 – y goes not according to the vacancy mechanism involving single vacancies but by cooperative motion of F– conduction ions.