Phenomenological Theoretical Study Of Ferroelectric Phase Transition In (111)-oriented Epitaxial PbTiO₃ Films

As a typical perovskite ferroelectric material,PbTiO₃(PTO)films have been studied in recent years.Most of these studies are based on changing the misfit strain or applying an external electric field to regulate the phase transition and properties of the films.However,recent studies have shown that the monoclinic structure of the fully clamped isotropic(111)-oriented PTO epitaxial films does not exist in the(001)-oriented films,and the monoclinic phase has been proved to have great piezoelectric response,which will promote the application of high-performance piezoelectric materials in electronic devices.In this paper,the phase transition and electrical properties of(111)-oriented PTO epitaxial films under compressive stress are studied by using the phenomenological theory of Landau-Ginzburg-Devenshire thermodynamics without considering or considering the depolarization field at room temperature.The main results are summary as follows:1.The stable phase structure and electrical properties of(111)-oriented single-domain PbTiO₃epitaxial films under compressive stress are obtained without considering the depolarization field at room temperature.The rhombohedral-monoclinic-paraelectric phase transition process under compressive stress shows that the stress can induce polarization rotation,promote spontaneous polarization to rotate into(111)plane,reduce spontaneous polarization,and inhibit the ferroelectricity of the films.The free energy profiles prove that two kinds of monoclinic phases can be stable in(111)PbTiO₃films which grow on compressive substrates,and may coexist in some condition.The curves of the relationship between polarization component,in-plane stress,out-of-plane strain,shear strain and compressive stress are obtained theoretically,which show that large spontaneous polarization,volume shrinkage and shear strain cannot be obtained simultaneously under compressive stress.In addition,The high piezoelectric and dielectric properties of(111)PbTiO₃films are gained by theoretical calculation in stable and metastable monoclinic phase under compressive stress.2.The phase transition and electrical properties of ultrathin(111)-oriented PbTiO₃epitaxial films under compressive stress have been studied at room temperature.Similar to the case when only the compressive stress is considered,the rhombohedral-monoclinic phase transition could also be induced under the depolarization field,but the compressive stress value corresponding to the phase transition is reduced by the depolarization field,and there are regions in which the two kinds of monoclinic phases independently exist.When depolarization field is considered,there will be no stable phase with out-of-plane polarization greater than zero,and there will also be stable phase only with in-plane polarization.The variation curves of in-plane stress,out-of-plane strain and shear strain also show that large spontaneous polarization,volume shrinkage and shear strain cannot be obtained simultaneously under depolarization field.The high dielectric constant and piezoelectric coefficient of the monoclinic phase induced by the depolarization field can be obtained by theoretical calculation,which is in good agreement with the results of the previous research on the monoclinic phase.