Phase transitions, stability, and dielectric response of the domain structure in ferroelectric-ferroelastic thin films.

The first analytical study of phase transitions and domain structure in ferroelastic-ferroelectric epitaxial thin films is presented for an exactly solvable model. The emerging domain structure with domains of equal width (which may be exponentially large) remains stable irrespective of the film thickness. Shifts of the domain walls, unexpectedly, contribute nothing (or insignificantly) to the dielectric response of the film. Generally, the motion of the domain walls results in about the same contribution to the response as the one that comes from a standard bulk term. Therefore, no particular softening of the dielectric response is expected to occur due to the motion of domain walls.