The Modulation Of Ferroelectric Properties Of Perovskites By The Coupling Between Strain And Lattice Distortion

Strain engineering plays an important role in tuning the properties of materials.The strong interplay between strain and lattice distortions in ABX3 perovskite gives rise to versatile intriguing multi-functional properties including ferroelectric,photovoltaic and multiferroic properties.The couplings between strain and lattice distortions could not only be used to rationalize fundamental sentific phenomenon but also could be utilized to design material for their technological applications in practical devices.Gaining insights into the microscopic physical mechanism behind strain-lattice distortion coupling and understand its rule in controlling properties of material is great importance in designing novel ferroelectric materials with multi-field couplings.Thus,the study of strain-lattice distortion coupling becomes the major research subjects of interdisciplinary sciences between mechanics,physics and material science.The thesis practically consist in the theoretical study of strain-ferroelectric distortion coupling,strain-octahedral rotation-ferroelectrice distortions coupling and strain-ferroelectric-Jahn-Teller distortions coupling in oxides,chalcogenide and halide perovskites through atomic scale first-principles and macroscopic scale landau theory.The objective of this thesis is to perform symmetry-adapted mode analysis and landau theory analysis to:i)explore the general mechanism behind strain-lattice distortion coupling,11)achieve desired ferroelectricity by strain-lattice distortion coupling and provide theoretical guidance for practical applications,?)activating particular multiple distortions coupling and realize the control of electronic properties by electric field.Several conclusions have been obtained as followings:Firstly,we perform a systematic study on the strain-ferroelectric distortion coupling in incipient ferroelectrics BaZrO3 and BaZrO3/SrZrO3 and explore the control of properties by activating such coupling.We demonstrate that the BaZrO3 crystal subjected to a critical compressive or tensile strain exhibits non-trivial spontaneous polarization that is higher than that of well-known ferroelectrics BaTiO3.Symmetry-adapted mode analysis show that strain-induced paraelectric-ferroelectric phase transition origins from strain triggered soften of ferroelectric lattice mode.At the same time,we give a further insight into the effect of strain on the dielectric properties and ferroelectric critical thickness of BaZrO3 thin film and the control of polarization in BaZrO3/SrZrO3 superlattice by strain.Due to the paraelectric-ferroelectric transition,the dielectric response of BaZrO3 thin film is significantly enhanced.In addition,we reveal that polarization can stably exist even in one-unit-cell thick BZO films,i.e.,absence of critical thickness.We demonstrate that there exists hybrid improper ferroelectricity in layered artificial BaZrO3/SrZrO3 stems from anti-ferrodistortive-ferroelectric distortions coupling and coexists with the strain-induced ferroelectric distortion.The coexistence of anti-ferrodistortive distortion and strain induced ferroelectric distortions results in an enhanced polarization in the superlattice.Secondly,through symmetry-adapted mode analysis and first-principles calculations,we develop a laudau equation by including the interactions between octahedral rotation,tilting,polar and anti-polar distortions in Ruddlesden-Popper(RP)A3B2X7 structure.We reveal that there exist strong anti-polar distortion at the AX interface when ABX3 coupounds form the A3BZX7 structure.It is found that the strong competition between anti-polar distortion and in-phase octahedral rotation is the physical mechanism that suppress the in-phase octahedral rotation.We firstly suggest to maintain the in-phase octahedral rotation by controlling chemical pressure in RP chalcogenide perovskites.There exist trilinear coupling between polarization,in-phase rotation and out-of phase tilting distortions in candidates with large chemical pressure,which gives rise to ferroelectricity.Then we propose there is strong coupling between strain and in-phase rotation,an experimentally accessible compressive strain could induce in-phase rotation and an in-plane polarization in RP perovskite halides thin films,resulting in an unusual paraelectric-ferroelectric phase transition.Through calculating the band gap by reliable hybrid Hartree-Fock density functional theory,we reveal that RP chalcogenide perovskites-with large chemical pressure and RP perovskite halide thin film not only have notable polarization but also possess suitable band gap,which make them optimal semiconductors for solar cell applications.Finally,we build a strain-ferroelectric-Jahn-Teller couplings physical model to study the complex strain-lattice distortions in BaFeO3/BaMnO3 superlattice and epitaxial BaSnO3 and SrSnO3 thin film.We perform a deep analysis on the coupling between lattice distortion and electronic structure and suggest a pathway to modulate the electronic properties through the electric field control of Jahn-Teller distortion.In particular,we discover strain-ferroelectric distortion and strain-Jahn-Teller distortion couplings in BaFeO3/BaMnO3 superlattice,simultaneously.Due to the strong electron-phonon coupling,we found a polar metal to insulating ferroelectrics transition under epitaxial strain.Moreover,there exist trilinear coupling between ferroelectric,Jahn-Teller and anti-polar distortions.Electric field is found to effectively tune the ferroelectric and Jahn-Teller distortions as well as the electronic structure and finally drives an insulator-metal transition.We reveal the different origin of Jahn-Teller distortion between BaSnO3 and SrSnO3 thin film and the BaFeO3/BaMnO3 superlattice even though the origin of ferroelectric polarization by strain-ferroelectric distortion coupling is the same in two system.We demonstrate the Jahn-Teller stems from three coupling forms,the coupling between ferroelectric and in-phase rotation distortions,the coupling between anti-polar and in-phase rotation distortions and the coupling between ferroelectric and anti-polar distortions,which give rise to the Jahn-Teller distortion.We found that Jahn-Teller distortion has significant effect on the band gap.In terms of the coupling between ferroelectric and Jahn-Teller distortion,we suggest it maybe possible to tune the band gap and absorption of sunlight by an external stimulus such as electric field.