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Dielectrics
Haven correlation parameter for diffusion of fluorine in superionic conductors La$_{1-y}$Sr$_{y}$F$_{3-y}$
N. I. Sorokin Shubnikov Institute of Crystallography, Federal Scientific Research Centre "Crystallography and Photonics", Russian Academy of Sciences, Moscow, Russia
Abstract:
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.
Received: 21.05.2018
DOI:
10.21883/FTT.2018.12.46723.102