Theoretical investigation of ferroelectric transitions in epitaxial ferroelectric films grown on a substrate

We have employed a pseudospin-phonon interaction model to investigate ferro electric transitions in epitaxial ferroelectric films on substrates. Using the double-time Green's function method, we have calculated a pseudospin-ph onon spectrum, effective pseudospin interaction constants and effective tun neling frequency. Numerical calculations give phase diagrams under various parameters. Without taking into account the pseudospin-phonon interaction i n the interface, the Curie temperature at the surface, T-c, was lower than that for the bulk, T-b Taking into account the pseudospin-phonon interactio n in the interface, T-c changed from the case of T-c > T-b to that of T-c < T-b with the layer number N <greater than or equal to> 2, leading to a cri tical value of the effective tunneling frequency. Under different effective tunneling frequencies, phase diagrams were obtained for various layers of the films. T-c changed from the case of T-c < T-b to that of T-c > T-b as N greater than or equal to 2, leading to a critical effective pseudospin int eraction in the interface. The lattice mismatch led to stresses in the inte rface between the film and substrate and led to changes in both the pseudos pin-phonon interaction and tunneling frequencies, thus altering the phase t ransition temperature of films. (C) 2000 Elsevier Science S.A. All rights r eserved.