Nuclear-magnetic-resonance study ofLi+motion in lithium aluminates and LiOH

We report $T_1$ and $T_2$ measurements of ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ resonances, which demonstrate thermally activated motion of ${\mathrm{Li}}^{+}$ in the ionic conductors $\alpha$- and $\beta$-${\mathrm{Li}}_5$Al${\mathrm{O}}_4$. At high temperatures our data give $\frac{T_1}{T_2}\approx {(I+\frac{1}{2})}^2=4\mathrm{}$, which demonstrates that dipole interactions between magnetic impurities and ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ produce the relaxation, even for our nominally pure samples, which are shown by ESR measurements to contain ∼ 15-ppm magnetic impurities. A liquidlike spatial diffusion model is used to analyze the relaxation times. At high temperatures the relaxation is determined by the ${\mathrm{Li}}^{+}$ jump time for which our analysis gives an activation energy $E_a=0.83\mathrm{}\left(2\right)\mathrm{}$ eV for $\alpha$, and $E_a=0.56\mathrm{}\left(4\right)\mathrm{}$ eV for $\beta$, with an attempt frequency ${\nu }_0=2\mathrm{}\times {10}^{12}$ ${\sec }^{-1\mathrm{}}$ for both phases. Near room temperature, $T_1$ is apparently determined by the impurity relaxation time, for which we estimate $T_{1e}\sim {10}^{-5\mathrm{}}$ sec. $T_2$ measurements in LiOH show motional narrowing of the ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ resonance above 320°C but not the ${}_{}{}^1{}_{}{}^{}\mathrm{H}$ resonance, and thus identify ${\mathrm{Li}}^{+}$ as the conducting ion in LiOH. Limited $T_1$ and $T_2$ measurements are also reported for ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ in $\gamma$-LiAl${\mathrm{O}}_2$.