Ferroelastic phase transitions in restricted crystals

Dependence of the value of the order parameter correlations radius on the crystal thickness in ferroelastic phase transitions is studied. It is shown that under certain conditions the striction coupling of the order parameter with lattice distortions can cause the spatially modulated structure to appear in the crystal, instead of homogeneous phase at a transition point, whose wave vector depends both on the constant of interaction between the order parameter and the elastic deformations as well as on the crystal plate thickness