  Electronic charge carriers in ionic materials can self-trap to form large
polarons. Interference between the ionic displacements associated with
oppositely charged large polarons increases as they approach one another.
Initially this interference produces an attractive potential that fosters their
merger. However, for small enough separations this interference generates a
repulsive interaction between oppositely charged large polarons. In suitable
circumstances this repulsion can overwhelm their direct Coulomb attraction.
Then the resulting net repulsion between oppositely charged large polarons
constitutes a potential barrier which impedes their recombination.
