The Energy Competitions And Domain Structures In Perovskite Ferroelectric Thin Films:Thermodynamics And Phase-field Simulations

It has been one hundred years since the discovery of ferroelectric materials.With the emergence of perovskite oxides represented by barium titanate and lead zirconate titanate from the 1950s,the study of ferroelectricity has gradually become a hot spot in the field of condensed matter physics.Traditionally,ferroelectric materials are widely used in sensors,memories,etc.by utilizing their ferroelectric and piezoelectric proper-ties.Nowadays,with the development of micro and nanotechnology,low-dimensional composite structures such as nanodots,nanotubes,superlattices,metamaterials,etc.demonstrate their unique and excellent performance.Therefore,they have broad appli-cation prospects.On the other hand,the thermodynamic phenomenological theory successfully ex-plained the macroscopic properties of ferroelectric materials in the 1960s.Further,the phase-field model based on the phenomenological theory predicted the formations,switching,and control of ferroelectric domains.Meanwhile,the first-principle calcula-tions,molecular dynamics and other methods deeply analyzed the physics backgrounds of ferroelectricity from a microscopic perspective.With the rapid development of high-performance computing,these multi-scale simulation methods are combined and be-come an indispensable part of the ferroelectric field now.Ferroelectric materials with the perovskite structure are the most investigated and most widely applied ferroelectrics.They are always the focus of researches.As the outstanding ones among them,ferroelectric thin films possess rich physical significa-tions.However,there are still many challenges for further explorations,e.g.the re-lationship between the formations of domain structures and the competitions among energies within ferroelectric thin films,the effects of the low-dimensional nature on the phase transitions,the designs of multi-functional devices based on thin-film struc-tures,etc.This thesis will select two typical perovskite ferroelectrics—barium titanate and bismuth ferrite—from the perspective of theoretical simulation,and perform the ther-modynamic analyses and phase-field simulations on the ferroelectric thin films.Due to the reduction of dimensions and the complication of the boundary conditions,the thin film structures should exhibit more features compared with their bulk counterparts,and lead to the enhancements of performances.The structure of the entire thesis is as fol-lows:Chapter 1 explains briefly the backgrounds of this thesis,including the defini-tion,properties,history,research progress,thin film structures,etc.of ferroelectrics.It follows various methods involved in ferroelectric field.In addition,this chapter also introduces the perovskite structures and some perovskite ferroelectric materials.Chapter 2 introduces the thermodynamic model and the phase field method of ferroelectric materials.The free energy terms of ferroelectrics including elastic en-ergy and electrostatic energy are given based on the Landau–Gingzburg–Denvonshire phenomenological theory.Meanwhile,this chapter introduces the phase-field model,including the derivations of numerical equations,the use of time-dependent evolution equations,and so on.Chapter 3 studies the domain boundaries of highly strained BiFeO₃films.Un-der high strain,BiFeO₃would produce a mixture of the super-tetragonal phase and the rhombohedral phase.This chapter summarizes all possible types of domain boundaries between above two phases,and calculated the directions of the domain boundaries with the lowest energy by thermodynamic methods.This chapter also analyzes the domain boundary energies based on the phase-field methods,and provided theoretical expla-nations for the behaviors of domain boundaries in the experiment.Chapter 4 studies the phase transitions of BiFeO₃ultrathin film.Due to the en-hancement of the interface effect,the original rhombohedral phase should gradually transfer to the purely tetragonal phase.Under certain substrate strains,the coexistence of the two phases(morphotropic phase boundaries)will appear,which greatly increases the piezoelectric performances of the materials.Chapter 5 studies the domain structures of the Ba Ti O₃microstructures.The top-down method can be used to engrave the nanopore arrays on the two-dimensional fer-roelectric thin films.Due to the competitions of the elastic energy and the electrostatic energy,the original striped domains can split.At the same time,the piezoelectric and dielectric effects will be enhanced as a result of the release of the above two energy terms.Chapter 6 contains the conclusions and the perspectives.