First-principles Studies On Multiferroic Properties And Structural Transitions Of Perovskite Oxides

In recent years,the perovskite structure oxides having the general chemical formula ABO₃ have received great attention because of the diverse properties,such as ferroelectricity,piezoelectricity,magnetic property,multiferroic property,giant magnetoresistance effect,photoelectric property,etc.and the large potential being used in the magnetic memory,detector sensors,solar cells and multifunctional materials.The multiferroic properties of perovskite structure oxides have also been the research hotspots for many years,to achieve the mutual coupling of ferroelectricity and magnetism,and to achieve the purpose of electronically control of magnetism,magnetically control of ferroelectricity and electromagnetic mutual control.In addition,due to the different growth environments,such as the temperature and pressure,and the preparation methods,perovskite materials always show different structural characteristics,which often leads to different physical and chemical properties.In this thesis,the first principles calculations and a first-principle-based effective Hamiltonian method are used to study the multiferroic properties of perovskite oxide materials and the structural phase transitions as a function of temperature.This paper is divided into five chapters,the main contents are as follows:In the first chapter,i.e.the introduction part,some basic structure information is firstly introduced for the perovskite material with processing molecular formula ABO₃,then briefly describes the magnetic properties,improper ferroelectricity and multiferroic properties in solid materials,and then briefly introduces the Landau theory of phase transition and the concept of the soft mode related to structural transition.The main research purposes,research significance,research objects and research contents of this thesis are expounded at the end of this chapter.The second chapter expounds the theoretical background and research methods used in this thesis,mainly the density functional theory and the first-principle-based effective Hamiltonian simulation method.In the third chapter,first-principles calculations are performed on magnetic multidomain structures in the SmFeO₃ rare-earth orthoferrite compound.We focus on the magnetic symmetry breaking at（001）-oriented antiphase domain walls,treating magnetism in the simplest（collinear）approximation without any relativistic（spin-orbit coupling）effects.We found that the number of FeO₂ layers inside the domains determines the electrical nature of the whole system:multidomains with odd number of layers are paraelectric,while multidomains with even number of layers possess an electric polarization aligned along b axis and a resulting multiferroic8)(82 ground state.Our ab initio data and model for ferroelectricity induced by spin order reveal that this polarization is of the improper type and originates from an exchange striction mechanism that drives a polar displacement of the oxygen ions located at the magnetic domain walls.Additional calculations ratify that this effect is general among magnetic perovskites with an orthorhombic SmFeO₃-like structure.In the fourth chapter,a first-principles-based effective Hamiltonian is developed and used,along with direct ab initio techniques,to investigate finite-temperature properties of the system commonly coined the most complex perovskite,that is NaNbO₃.Such simulations successfully reproduce the existence of seven different phases in its phase diagram.The decomposition of the total energy of this effective Hamiltonian into different terms,altogether with the values of the parameters associated with these terms,also allow us to shed some light into puzzling features of such a compound.Examples include revealing the microscopic reasons of why3(8 is its ground state and why it solely adopts in-phase tiltings at high temperatures versus complex nanotwins for intermediate temperatures.The results of the computations also call for a revisiting of the so-called P,R,and S states,in the sense that an unexpected and previously overlooked inhomogeneous electrical polarization is numerically found in the P state while complex tiltings associated with the simultaneous condensation of several k points are predicted for the controversial R and S phases.In the fifth chapter,the research contents of this thesis are summarized.Some research prospections are pointed out,which are about the generation of improper ferroelectricity,the discovery of multiferroic properties and the use of the first-principle-based effective Hamiltonian method for NaNbO₃ system and other systems.