Heterostructured (β/γ)-AgI exhibits a spontaneous lattice polarization not accounted for in standard space-charge models. This polarization field dominates the positional variation of energies of isolated defects, and Ag+ vacancies and interstitials are stabilized at alternate [β/γ] interfaces. This suggests enhanced Frenkel pair separation, analogous to electronic charge separation in polar semiconductor heterostructures. Stoichiometric systems are, however, characterized by associated Frenkel pairs due to strong VAg—Agi interactions and show no enhancement of defect numbers. In nonstoichiometric systems, lattice polarization does direct the distribution of the excess defect species, and defect–defect interactions enhance local Frenkel pair concentrations at interfaces, suggesting that nonstoichiometry is critical to the extreme room-temperature ionic conductivities observed in heterostructured AgI nanoplates.