Vacancy distribution and ionic motion inLaF3studied by19F NMR

We report on ${}_{}{}^{19}{}_{}{}^{}$F pulse-NMR experiments on a single crystal of the fast ionic conductor ${\mathrm{LaF}}_3$ from 100 to 1250 K. The spin-lattice relaxation time $T_1$ is measured at frequencies up to 140 MHz, and its counterpart in the rotating frame $T_{1\rho }$, in spin-locking fields up to 12 G. In the regime of composite free-induction decays two spin-spin relaxation times $T_2$ are extracted. The data are analyzed in terms of the equations of motion of the nuclear magnetizations on two fluorine sublattices, $F_1$ and $F_2$,3, with inclusion of fluorine-fluorine dipolar interactions and relaxation to paramagnetic impurities. To fit all data simultaneously, it appeared essential to assume an energy difference Q between the depth of the potential wells at the $F_1$ and $F_2$,3 sites in the sense that the anion vacancies preferentially populate the $F_2$,3 sublattice. The model then gives realistic values for the activation energies and attempt frequencies for jumps within and between the $F_1$ and $F_2$,3 sublattices, while Q=0.119±0.005 eV. Above 400 K, the vacancies occupy, with an increasing concentration, the $F_1$ positions, where the residence time is longer than on the $F_2$,3 sites. The model also accounts for the knee observed in the conductivity of ${\mathrm{LaF}}_3$-structured materials versus reciprocal temperature on the basis of the defect-defect interaction between trapped $F_1$ vacancies and $F_2$,3 vacancies carrying the ionic conductivity.