BiFeO3中各离子在铁电相变中作用本质的第一性原理研究

采用基于密度泛函理论的广义梯度近似方法和赝势平面波法，对多铁材料BiFeO₃的铁电反铁磁相和可能的高温顺电相的电子结构进行了第一性原理研究.计算验证了BiFeO₃基态为G型反铁磁有序，Fe离子的理论磁矩与实验值相符.铁电相变发生后，Bi-6s态和6p态发生了电荷转移，Bi-6s电子的作用更加明显.Born有效电荷的研究表明铁电畸变主要表现为Bi原子的位移，并且电极化强度计算值很好地符合薄膜实验结果.部分态密度的计算表明Bi-6p态的成键轨道与反键轨道间的能量劈裂 关键词： 第一性原理 铁电性 铁电畸变 反铁磁性

First-principles investigation on the role of ions in ferroelectric transition of BiFeO3

The electronic properties of multiferroic BiFeO₃ with paraelectric paramagnetic phase at high-temperature and ferroelectric antiferromagnetic phase are calculated using density functional theory with spin-polarized generalized-gradient approximation (GGS) and plane wave pseudopotentials. study of Born effective charges shows that the greatest contributor to the ferroelectric distortion is Bi atoms' displacement. We obtained very large theoretical electric polarization, which agrees very well with the results of thin-film test. The calculation shows that BiFeO₃ ground state has G type anti-ferromagnetic order and the theoretical magnetic moments of Fe ion accord with experimental values. After ferroelectric phase transformation, the chemical bond changes much due to Bi-6s and Bi-6p state charge transfer, and the effect of Bi-6s electrons becomes more pronounced. Partial density of states calculation indicates that the energy split between bonding orbit of Bi-6p state and anti-bonding orbit is bigger than that between other electronic states, by which covalent bond between Bi-6p state and O-2p state is strengthened, which is the origin of off-center displacement. Thus the discussion about the BiMnO₃ is testified. Bi-6s state is polarized due to the static electric repulsion, but does not take part in the hybridization of Bi-6p and O-2p electrons. The charge transfer originates from its weak covalent interaction with the O-2s, 2p orbits, by which the Bi-6s state electronic polarization is reinforced. This kind of covalent interaction helps the relative replacement of Bi-O, which is the reason for the strong systemic electric polarization.