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

Resumo

The processes of electric charge transfer (conductivity) and mass transfer (diffusion) in La_1 ‒ ySr_yF_3 – y superionic conductors are determined by mobile fluorine ions. The fluorine random-diffusion coefficients D_σ have been calculated from experimental data on the ionic conductivity σ_dc for La_1 ‒ ySr_yF_3 – 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 D_σ(y). The value of the Haven correlation parameter H_r = D_NMR/D_σ (D_NMR is the fluorine diffusion coefficient measured by NMR on ¹⁹F nuclei) is determined; it characterizes the ion transport mechanism in La_1 – ySr_yF_3 – y crystals. The H_r 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 La_0.97Sr_0.03F_2.97 with maximum σ_dc and D_σ values has a minimum Haven parameter. The obtained H_r values indicate that fluorine diffusion in superionic conductors La_1 – ySr_yF_3 – y goes not according to the vacancy mechanism involving single vacancies but by cooperative motion of F^– conduction ions.