  While it is well established that ionic conduction in lithium
aluminosilicates proceeds via hopping of Li ions, the nature of the various
hoping-based mechanisms in different temperature regimes has not been fully
elucidated. The difficulties associated with investigating the conduction have
to do with the presence of grains and grain boundaries of different
orientations in these usually polycrystalline materials. Herein, we use
electrochemical impedance spectroscopy (EIS) to investigate the ion conduction
mechanisms in -eucryptite, which is a prototypical lithium aluminosilicate. In
the absence of significant structural transitions in grain boundaries, we have
found that there are three conduction regimes for the one-dimensional ionic
motion along the c axis channels in the grains, and determined the activation
energies for each of these temperature regimes. Activation energies computed
from molecular statics calculations of the potential energy landscape
encountered by Li ions suggest that at temperatures below 440 {\deg}C
conduction proceeds via cooperative or correlated motion, in agreement with
established literature. Between 440 {\deg}C and 500{\deg}C, the activation
barriers extracted from EIS measurements are large and consistent with those
from atomistic calculations for uncorrelated Li ion hopping. Above 500 {\deg}C
the activation barriers decrease significantly, which indicates that after the
transition to the Li-disordered phase of -eucryptite, the Li ion motion largely
regains the correlated character.
