THERMODYNAMICS OF SINGLE-DOMAIN BATIO3 AND PBTIO3 EPITAXIAL THIN-FILMS

A phenomenological thermodynamic theory of ferroelectric thin films ep itaxially grown on cubic substrates is developed using a new form of t he Gibbs function, which corresponds to the actual mechanical boundary conditions of the problem. It is found that the 2D-clamping of the fi lms results not only in a shift of the transition temperature but also in a change of its order. For single-domain BaTiO3 and PbTiO3 films, the ''misfit strain-temperature'' phase diagram is constructed. For Pb TiO3 films, the existence of a phase forbidden for the crystals is pre dicted. The intrinsic piezoelectric response of BaTiO3 films is calcul ated as a function of the misfit strain.