Non-Arrhenius conductivity in the fast lithium conductor Li1.2Ti1.8Al0.2(PO4)3 : A Li7 NMR and electric impedance study

Lithium ion motion has been investigated in the ${\mathrm{Li}}_{1.2}{\mathrm{Ti}}_{1.8}{\mathrm{Al}}_{0.2}{(\mathrm{P}{\mathrm{O}}_{4})}_{3}$ compound with NMR and impedance spectroscopies. From the analysis of the bulk dc conductivity $({\ensuremath{\sigma}}_{b,\mathrm{dc}})$ and the frequency at the maximum $({\ensuremath{\omega}}_{bp})$ of the imaginary electric modulus, two regimes for lithium motion have been identified. From the analysis of the temperature dependence of $^{7}\mathrm{Li}$ NMR quadrupole constant $({C}_{Q})$ and spin-spin relaxation rate $({T}_{2}^{\ensuremath{-}1})$, the evolution of ${M}_{1}$ and ${M}_{2}$ sites occupancy in the conduction network of the fast ion conductor has been deduced. At low temperatures $(T<250\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, differences in activation energies deduced from NMR $({E}_{m}^{R}=0.19\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ and conductivity $({E}_{M}^{C}=0.31\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ are discussed in terms of a correlated lithium motion. In this regime, the $\ensuremath{\beta}$ parameter that relates both energies $({E}_{m}=\ensuremath{\beta}{E}_{M})$ takes the value 0.61. In the high temperature regime $(T>250\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, the activation energy ${E}_{M}$ deduced by the two techniques show similar values $(0.22\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$. In this case the $\ensuremath{\beta}$ parameter is nearly equal to 0.85. From the NMR results, it has been concluded that the partial cancellation of the correlation in lithium motion is associated with the creation of vacancies at ${M}_{1}$ sites. On the other hand, some differences in $\ensuremath{\beta}$ and ${\ensuremath{\tau}}_{0}$ parameters have been detected, indicating that relaxation functions are slightly different in two techniques. In particular, the NMR ${\ensuremath{\beta}}^{R}$ correlation factor is slightly higher and the NMR ${\ensuremath{\tau}}_{0}^{R}$ parameter is almost one order of magnitude higher than that deduced from electric measurements.