第一性原理论文：BiFeO_3结构与性能的第一性原理研究
【中文摘要】多铁材料是一种同时具有铁电性、铁磁性或铁弹性的材料,作为新型多功能材料,它可以用来转换能量,传感信号,检测磁（电）场等。BiFeO3是一种典型的单相多铁材料,研究表明BiFeO3的铁电极化起源于Bi3+的6s2孤对电子,但它的铁磁性就比较复杂,并非简单的反铁磁有序,而是具有空间调制的螺旋磁结构。正是由于其结构的特殊性,而引起了人们广泛地研究。本文利用第一性原理研究BiFeO3的结构与性能,主要内容如下:1.不同的强关联势对BiFeO3结构的影响;从电荷密度和态密度分析出BiFeO3各态之间的相互作用,并给出铁电性的起因。BiFeO3存在着两种扭曲形式,在方向上Bi3+相对FeO6八面体位移和FeO6八面体沿极化方向上的反铁扭曲旋转,通过这两种扭曲方式衍生出多种结构,并得出七种不同空间群的BiFeO3结构特征（R3c, R3m, P4mm, Cm, Fm3m, R3m和R3c）。2.引入两个共顶点四面体模型更加清晰地描述BiFeO3的铁电转变。在铁电转变过程中,顺电相的能量、原子占位和极化极化强度都存在突变,对于在反铁磁相下顺电相的O原子磁矩具有微小的自旋阻挫,这是导致铁电转变过程中出现突变的原因。在顺磁相中铁电相和顺电相之间的能量差为140 meV左右,估算出铁电转变温度TC大约为1500 K。分别用玻恩有效电荷和贝里相位计算得出其铁电极化强度分别为86.2和98.0μC/cm2。3.总结四方相（P4mm） BiFeO3的基本性质,从态密度和玻恩有效电荷中得到四方相的的铁电性是由Fe原子和顶
角O原子之间成键引起的,采用贝里相位方法得出稳定的四方相极化强度为152.76μC/cm2。采用海森堡交换模型分析应变对磁有序的影响,在较小的晶格常数下,C-AFM比较稳定,随着晶格常数的增加,其磁有序转变为G-AFM。还得出随着晶格常数a的增加,c/a比值相应变小,而其极化强度却逐渐减小。最后构造四方相方向的BiFeO3表面模型,通过比较界面能和电荷密度的分布情况得出FeO2的截面要比BiO的稳定。
【英文摘要】The multiferroic materials have been coined to describe materials which have simultaneous ferroelectricity, ferromagnetism or ferroelasticity. As new multifunctional materials, the potential applications include energy conversion, sensing signal and detection of magnetic （electric） field. BiFeO3 is a typical single-phase multiferroic material. The research shows that the origin of ferroelectricity is due to the Bi-6s2 lone pairs. The ferromagneticity is more complicated. It is not a simple antiferromagnetic order, but it has the helical magnetic structure of spatial modulation. BiFeO3 has attracted extensive research because of its special structure.In this thesis, our purpose is to study the structure properties of BiFeO3 with the help of the first-principles study. The following major elements include:1. The structures of BiFeO3
are influenced by different strong correlation potential. The interaction among different states is summarized by charge density and density of states, and the origin of
ferroelectricity is also concluded. There are two types of structural distortions, the relative displacement between FeO6 ocahedra and Bi3+ along [111] and the rotation of the FeO6 ocahedra about the [111] axis. Several structures can be derived from these two structural distortions. We investigate the structural characteristics of seven different phases of BiFeO3 including R3c, R3m, P4mm, Cm, Fm3 m, R3 m and R3 c.2. We introduce corner-sharing double-tetrahedrons of FeO6 unit to describe the ferroelectric switching in the rhombohedral phase of BiFeO3. During the ferroelectric switching we find abrupt changes in the atomic positions, total energy and ferroelectric polarization in the paraelectric phase. In the antiferromagnetic phase the magnetic moments of O atoms have spin frustrations in the paraelectric phase, giving rise to abrupt changes. The energy difference between the
ferroelectric ground state and the paraelectric state with paramagnetic phase is about 140 meV, which corresponds to the transition temperature, 1500 K. The ferroelectric polarization is obtained, 86.2 and 97.6μC/cm2 by born effective charges and
Berry phase, respectively.3. The properties of tetragonal BiFeO3 （P4mm） is summarized in detail. The ferroelectric of tetragonal structure originates from the covalent bond between apical O and Fe by born effective charges and density of states （DOS）. The ferroelectric polarization calculated by Berry phase is 152.76μC/cm2. The Heisenberg model is applied to analyze how the strain influences the magnetic ordering. C-type antiferromagnetic is stable in the low in-plane lattice constant. There is a transformation in the magnetic ordering associated with in-plane lattice parameter increases, from C-type to G-type antiferromagnetic. With increasing the in-plane lattice parameter the ferroelectric polarization and c/a can be correspondingly decreased. At last the BiFeO3 （001） surface of P4mm is built to study the section property. The FeO2 interface is more stable than BiO interface by interfacial energy and charge density.
【关键词】第一性原理 BiFeO3 电子结构 结构转变 铁电转变 磁有序
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【英文关键词】First-principles calculation BiFeO3
Electronic structure Structural transition Ferroelectric switching Magnetic ordering
【目录】BiFeO_3结构与性能的第一性原理研究4-510-12
ABSTRACT5-6
第一章 绪论10-27
12-16
摘要1.1 前言1.2.1
1.2 BiFeO_3 的性能与特点
12
BiFeO_3 的晶体结构12-13
1.2.2 BiFeO_3 的铁电性
1.2.4 BiFeO_3
1.3.1
1.2.3 BiFeO_3 的铁磁性13-14
14-1617-18
其它丰富的物理特性BiFeO_3 的实验制备18-1921-27
1.3 研究现状16-191.3.2 计算研究进展
1.4 本文研究的目的及意义19-21参考文献
2.1
第二章 第一性原理计算方法概述27-39
2.1.1 绝热近似
28-29
Hartree-Fock 方法27-2927-28
2.1.2 Hartree-Fock 近似2.2 密度泛函
理论简介29-3229
2.2.1 Thomas-Fermi-Dirac 近似
2.2.3 Kohn-Sham 方
2.2.2 密度泛函理论定理29
程29-3131-3233-3434
2.2.4 局域密度近似和广义梯度近似2.3 平面波方法32-332.4.1 赝势33-34
2.4 赝势与PAW 方法2.4.2 投影缀加波方法参考文献36-393.1 计算软件与细节3.1.2 计算细节
3.2.1 强第三章
2.5 结构优化34-36
BiFeO_3的基本结构性质39-5439-4039-40
3.1.1 VASP 简介39
3.2 R3C 结构的BiFeO_3 性质40-46
第一性原理论文：BiFeO_3结构与性能的第一性原理研究
