  Polycrystalline solids can exhibit material properties that differ
significantly from those of equivalent single-crystal samples, in part, because
of a spontaneous redistribution of mobile point defects into so-called
space-charge regions adjacent to grain boundaries. The general analytical form
of these space-charge regions is known only in the dilute limit, where
defect-defect correlations can be neglected. Using kinetic Monte Carlo
simulations of a three-dimensional Coulomb lattice gas, we show that
grain-boundary space-charge regions in non-dilute solid electrolytes exhibit
overscreening -- damped oscillatory space-charge profiles -- and underscreening
-- decay lengths that are longer than the corresponding Debye length and that
increase with increasing defect-defect interaction strength. Overscreening and
underscreening are known phenomena in concentrated liquid electrolytes, and the
observation of functionally analogous behaviour in solid electrolyte
space-charge regions suggests that the same underlying physics drives behaviour
in both classes of systems. We therefore expect theoretical approaches
developed to study non-dilute liquid electrolytes to be equally applicable to
future studies of solid electrolytes.
