  A thermodynamic theory is developed for dense laminar domain structures in
epitaxial ferrolectric films. It is found that, at some critical misfit strain
between the film and substrate, the 90 degrees c/a/c/a domain structure becomes
unstable with respect to the appearance of the polarization component parallel
to domain walls, which results in the formation of a heterophase structure. For
PbTiO_3 and BaTiO_3 films, the stability ranges of polydomain and heterophase
states are determined using "misfit strain - temperature" diagrams. Dielectric
anomalies accompanying misfit-strain-driven structural transformations are
described.
