Phenomenological Theoretical Study of Ferroelectric Phase Transitions in Epitaxial Perovskite Thin Films
|Course||Materials Physics and Chemistry|
|Keywords||Ferroelectric thin films Phase transition Epitaxial strain Phenomenological theory|
Ferroelectric thin films have attracted considerable research interest due to their superior dielectric, piezoelectric, pyroelectric and ferroelectric properties and potential applications in microelectronic and electric-optic devices. Epitaxial strains can have a substantial impact on structure, phase transition, and dielectric and piezoelectric responses in perovskite ferroelectric thin films. In this paper, we investigate the effects of epitaxial strains on ferroelectric phase transitions and physical properties in epitaxial perovskite thin films by utilizing phenomenological Landau-Ginzburg-Devonshire theory. The main contents and results are given as follow.1. For KNbO3 thin films epitaxially grown on cubic substrates, isotropic biaxial misfit strains lead to emergence of paraelectric tetragonal phase (p), ferroelectric tetragonal phase (c), orthorhombic phase (aa), and monoclinic phase (r), and all the phase transitions are of the second-order. However, paraelectric cubic phase, ferroelectric_tetragonal phase, orthorhombic phase and rhombohedral phase can only exist in KNbO3 bulk materials, and all the phase transitions are of the first-order. Both positive and negative misfit strains lead to an increase in the Curie temperature. The dielectric constantsε11 andε33 approach infinity at the c-r and r-aa transitions, respectively.2. For KNbO3 thin films epitaxially grown on orthorhombic substrates, anisotropic biaxial misfit strains result in the appearance of orthorhombic phases (a1, a2) and monoclinic phases (a1c, a2c). It is worthwhile to note that a1, a2, a1c, and a2c phases do not exist in KNbO3 thin films epitaxially grown on cubic substrates. The anisotropic biaxial misfit strains lower symmetry of the paraelectric phase from tetragonal to orthorhombic, so symmetry of ferroelectric phases is also lowered. Anisotropic biaxial misfit strains transform the aa phase and r phase formed under isotropic biaxial strains into a1a2 phase (spontaneous polarization Ps rotated away from in-plane face diagonal of the unit cell) and r’ phase (Ps is rotated away from (110) face of the unit cell), respectively. Anisotropy of biaxial misfit strains can also lead to the dielectric anisotropy.3. In-plane shear strain is shown to restrain formation of a phase (polarization along ). The in-plane shear strain increases the p-aa transition temperature and shifts the c-r and r-aa phase boundaries to the compressive strain direction. In-plane shear strain increases the in-plane spontaneous polarization but reduces the out-of-plane spontaneous polarization. Accordingly, in-plane shear strain improves dielectric responseε33 in r phase, and decreases dielectric responseε33 in aa phase. If a12*> 2a11*> 0, a phase can appear stably in the narrow range between p and aa phase, and ac phase can exist in the narrow range between c and r phase in the case of isotropic strains.