  The standard model of enhanced ionic conductivities in solid electrolyte
heterostructures follows from a continuum mean-field description of defect
distributions that makes no reference to crystalline structure. To examine
ionic transport and defect distributions while explicitly accounting for
ion-ion correlations and lattice effects, we have performed molecular dynamics
simulations of a model coherent fluorite heterostructure without any extrinsic
defects, with a difference in standard chemical potentials of mobile fluoride
ions between phases induced by an external potential. Increasing the offset in
fluoride ion standard chemical potentials across the internal interfaces
decreases the activation energies for ionic conductivity and diffusion and
strongly enhances fluoride ion mobilities and defect concentrations near the
heterostructure interfaces. Non-charge-neutral "space-charge" regions, however,
extend only a few atomic spacings from the interface, suggesting a continuum
model may be inappropriate. Defect distributions are qualitatively inconsistent
with the predictions of the continuum mean-field model, and indicate strong
lattice-mediated defect-defect interactions. We identify an atomic-scale
"Frenkel polarisation" mechanism for the interfacial enhancement in ionic
mobility, where preferentially oriented associated Frenkel pairs form at the
interface and promote local ion mobility via concerted diffusion processes.
