Proper ferroelastic phase transitions in thin epitaxial films with symmetry-conserving and symmetry-breaking misfit strains - art. no. 134107

We study how the ferroelastic domain structure sets in in an epitaxial film of a material with second-order proper ferroelastic transition. The domain structures considered are similar to either a(1)/a(2)/a(1)/a(2) or c/a/c/a structures in perovskite ferroelectrics. If the "extrinsic" misfit strain, not associated with the transition, does not break the symmetry of the hig h-temperature phase, the phase transition in the film occurs at somewhat lo wer temperature compared to the bulk. The loss of stability then occurs wit h respect to a sinusoidal strain wave, which evolves into the domain struct ure with practically the same geometry and approximately the same period. I n the presence of the symmetry-breaking component of the misfit strain ("ex trinsic" misfit) the character of the phase transition is qualitatively dif ferent. In this case it is a topological transition between single-domain a nd multidomain states, which starts from a low density of the domain walls.