  Li$_{10}$Ge(PS$_6$)$_2$ (LGPS) is a highly concentrated solid electrolyte, in
which Coulombic repulsion between neighboring cations is hypothesized as the
underlying reason for concerted ion hopping, a mechanism common among
superionic conductors such as Li$_7$La$_3$Zr$_2$O$_{12}$ (LLZO) and
Li$_{1.3}$Al$_{0.3}$Ti$_{1.7}$(PO$_4$)$_3$ (LATP). While first principles
simulations using molecular dynamics (MD) provide insight into the Li$^+$
transport mechanism, historically, there has been a gap in the temperature
ranges studied in simulations and experiments. Here, we used a neural network
(NN) potential trained on density functional theory (DFT) simulations, to run
up to 40-nanosecond long MD simulations at DFT-like accuracy to characterize
the ion conduction mechanisms across a range of temperatures that includes
previous simulations and experimental studies. We have confirmed a Li$^+$
sublattice phase transition in LGPS around 400 K, below which the
\textit{ab}-plane diffusivity $D^*_{ab}$ is drastically reduced. Concomitant
with the sublattice phase transition near 400 K, there is less cation-cation
(cross) correlation, as characterized by Haven ratios closer to 1, and the
vibrations in the system are more harmonic at lower temperature. Intuitively,
at high temperature, the collection of vibrational modes may be sufficient to
drive concerted ion hops. However, near room temperature, the vibrational modes
available may be insufficient to overcome electrostatic repulsion, thus
resulting in less correlated ion motion and comparatively slower ion
conduction. Such phenomena of a sublattice phase transition, below which
concerted hopping plays a less significant role, may be extended to other
highly concentrated solid electrolytes such as LLZO and LATP.
