Ferroelectric thin films and multilayer structures based on them

A brief review is presented of rite results of recent research into ferroel ectric films and their multilayer structures. The main attention is paid to theoretical calculation of the physical properties that characterize ferro electric materials (electric polarization, Phase transition temperature, di electric response) in thick and thin films and their multilayer structures. Within the phenomenological theory it is shown that the main reason for a decrease in film symmetry Is internal mechanical stress connected with the mismatch in lattice constants, difference in thermal expansion coefficients of the substrate mid film, and also growth imperfections. These stresses l ead to a change (decrease or increase) in the para-ferroelectric transition temperature that is actually observed in thick film. In thin films, where if is necessary to consider polarization gradients, a ferroelectric transit ion develops whose temperature depends on film thickness (thickness induced phase transition). The polarization and dielectric permittivity of films a nd their multilayer-structures are calculated It is demonstrated that permi ttivity becomes infinitely great close to the thickness induced phase trans ition temperature. The theory fits well with the recently observed huge die lectric permittivity and its temperature dependence in a multilayer structu re of thin films of PbTiO3 and Pb(0.72)Ln(0.28)TiO(3).