Ferroelectric Phase Transitions and Domain Structure Evolution in Strained Thin Films – Thermodynamic Theory and Phase-field Simulations

Phase transition temperatures and microstructure evolution in thin films under strain could be dramatically different from the corresponding unconstrained bulk single crystals. As an example, BaTiO₃ ferroelectic thin films under a substrate constraint are studied using the phenomenological Landau theories and phase-field simulations. It is predicted that ferroelectric transition temperatures can be shifted by as much as 300C for fully coherent (001) BaTiO₃ thin films epitaxially grown on (110) GdScO₃ and about 450C for (001) BaTiO₃ films on (110) DyScO₃ as compared to the bulk ferroelectric transition temperature. The results are validated by high-temperature x-ray diffraction and second harmonic generation (SHG) measurements. Based on the phase-field simulations, a domain stability map was constructed for BaTiO₃ thin films, which displays the stability of various ferroelectric phases and domain structures as a function of temperature and magnitude of substrate constraint.