Haven correlation parameter for diffusion of fluorine in superionic conductors La$_{1-y}$Sr$_{y}$F$_{3-y}$

The processes of electric charge transfer (conductivity) and mass transfer (diffusion) in La$_{1-y}$Sr$_{y}$F$_{3-y}$ superionic conductors are determined by mobile fluorine ions. The fluorine random-diffusion coefficients $D_\sigma$ have been calculated from experimental data on the ionic conductivity $\sigma_{dc}$ for La$_{1-y}$Sr$_{y}$F$_{3-y}$ single crystals at the SrF$_2$ dopant contents of 1, 3, 5, 7.5, 10, and 15 mol %. A maximum in the region of 3–5 mol % SrF$_2$ is observed in the dependence $D_\sigma(y)$. The value of the Haven correlation parameter $H_r=D_{NMR}/D_\sigma$ ($D_{NMR}$ is the fluorine diffusion coefficient measured by NMR on $^{19}$F nuclei) is determined; it characterizes the ion transport mechanism in La$_{1-y}$Sr$_{y}$F$_{3-y}$ crystals. The $H_r$ values are 0.75 $\pm$ 0.15, 0.45 $\pm$ 0.15, and 0.65 $\pm$ 0.15 at 400–800 K for 1, 3, and 15–16 mol % SrF$_2$, respectively. Superionic conductor La$_{0.97}$Sr$_{0.03}$F$_{2.97}$ with maximum $\sigma_{dc}$ and $D_\sigma$ values has a minimum Haven parameter. The obtained $H_r$ values indicate that fluorine diffusion in superionic conductors La$_{1-y}$Sr$_{y}$F$_{3-y}$ goes not according to the vacancy mechanism involving single vacancies but by cooperative motion of F$^-$ conduction ions.