[论文解读] Domain walls in a perovskite oxide with two primary structural order parameters: first-principles study of BiFeO$_3$

Oswaldo Di 'eguez, Pablo Aguado‐Puente+2 more|LA Referencia (Red Federada de Repositorios Institucionales de Publicaciones Científicas)|Nov 21, 2012

Multiferroics and related materials被引用 6

一句话总结

这项从头算研究揭示，在多铁性材料BiFeO₃中，铁电畴壁（FE-DWs）的稳定性主要由反铁畸变（AFD）氧八面体旋转模式决定，而非极化变化。与传统假设相反，AFD序参量的不连续性主导了畴壁的能级，能量最低的畴壁呈现出低激发亚稳相的原子结构特征，表明BFO在纳米尺度畴边界处表现出多型材料的特性。

ABSTRACT

We present a first-principles study of ferroelectric domain walls (FE-DWs) in multiferroic BiFeO$_3$ (BFO), a material in which the FE order parameter coexists with anti-ferrodistortive (AFD) modes involving rotations of the O$_6$ octahedra. We find that the energetics of the DWs are dominated by the capability of the domains to match their O$_6$ octahedra rotation patterns at the plane of the wall, so that the distortion of the oxygen groups is minimized. Our results thus indicate that, in essence, it is the discontinuity in the AFD order parameter, and not the change in the electric polarization, what decides which crystallographic planes are most likely to host BFO's FE-DWs. Such a result clearly suggests that the O$_6$ rotational patterns play a primary role in the FE phase of this compound, in contrast with the usual (implicit) assumption that they are subordinated to the FE order parameter. Our calculations show that, for the most favorable cases in BFO, the DW energy amounts to a several tens of mJ/m$^2$, which is higher than what was computed for other ferroelectric perovskites with no O$_6$ rotations. Interestingly, we find that the structure of BFO at the most stable DWs resembles the atomic arrangements that are characteristic of low-lying (meta)stable phases of the material. Further, we argue that our results for the DWs of bulk BFO are related with the nanoscale-twinned structures that Prosandeev et al. [Adv. Funct. Mats. (2012), doi: 10.1002/adfm.201201467] have recently predicted to occur in this compound, and suggest that BFO can be viewed as a polytypic material. Our work thus contributes to shape a coherent picture of the structural variants that BFO can present and the way in which they are related.

研究动机与目标

• 理解BiFeO₃中铁电畴壁（FE-DWs）的原子尺度能量特性，该材料是一种具有共存铁电（FE）和反铁畸变（AFD）序参量的多铁性钙钛矿材料。
• 通过研究AFD畸变在决定畴壁能量和结构中的作用，挑战传统认为FE序参量主导畴壁稳定性的假设。
• 探讨稳定畴壁的原子结构与已知BiFeO₃低激发亚稳相之间的关联。
• 评估这些发现对解释BiFeO₃薄膜中纳米尺度孪生结构实验观测结果的启示。
• 为建模具有竞争序参量的复杂氧化物中畴壁行为提供理论基础，超越理想化模型。

提出的方法

• 采用密度泛函理论（DFT）计算，模拟同时具有FE极化和AFD八面体旋转模式不连续性的混合畴壁。
• 使用超原胞模拟与周期性边界条件，计算块体BiFeO₃中各种畴壁构型的总能量。
• 系统改变畴壁的晶体取向和对称性，以识别能量最低的构型。
• 分析畴壁处的原子位移、键长和旋转畸变，以表征结构变化。
• 将最稳定畴壁的原子结构与已知的BiFeO₃亚稳相进行比较，识别结构相似性。
• 评估畴壁能量随畴壁间距增加的收敛性，以确认理想体系中不存在长程相互作用。

实验结果

研究问题

• RQ1 当铁电和反铁畸变序参量同时存在时，是什么决定了BiFeO₃中不同铁电畴壁的相对稳定性？
• RQ2 与极化反转相比，氧八面体旋转（AFD模式）在多大程度上影响畴壁的能量和原子结构？
• RQ3 BiFeO₃中最稳定畴壁的原子结构能否与已知的低激发亚稳相相对应？
• RQ4 块体BiFeO₃中畴壁计算结果如何与薄膜中实验观测到的纳米尺度孪生结构相关联？
• RQ5 为何BiFeO₃薄膜中畴壁的实验观测结果与基于块体能量计算的理论预测存在差异？

主要发现

• BiFeO₃中铁电畴壁的能量主要由反铁畸变（AFD）序参量的不连续性决定，而非铁电极化的变化。
• 能量最低的畴壁表现出与已知BiFeO₃低激发亚稳相极为相似的原子结构，表明存在直接的结构关联。
• BiFeO₃中畴壁的能量在几十mJ/m²量级，显著高于无AFD畸变的典型钙钛矿铁电体。
• 最稳定的畴壁为原子级薄，宽度约为1 nm，与其它钙钛矿材料的从头算研究结果一致。
• 结果表明，BiFeO₃可被视为一种多型材料，其中畴边界对应于不同（亚）稳定多型之间的转变。
• 理论预测与薄膜中实验观测之间的差异可能源于应变、缺陷、界面和电极效应等非本征因素，这些因素在块体模拟中未被考虑。