First-principles Investigation On The Structures And Properties Of BiFeO₃

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.