NMR and XAS study on doped $\mathrm{LaF_3}$

Temperature dependent NMR measurements as well as XAS investigations on $La_{1−x}Sr_xF_{3−x}$ (x=0.0, 0.01, 0.05, 0.16) have been performed to study the fluorine diffusion in these solid electrolytes and to elucidate the effect that doping with heterovalent $SrF_2$ has on $LaF_3$. A three-stage process of diffusion was observed. Upon increasing temperature, initial ionic exchange begins in the $F_1$ sublattice. At even higher temperatures, dependent upon dopant concentration, ionic exchange between the F$_1$ and F$_{2,3}$ sublattices takes place. A further increase in temperature eventually leads to exchange over all F$^−$ structural positions