【纯计算】我怎么就丢了铁电性？

近年来，化学式为ABX₃的有机金属卤化物钙钛矿，特别是铅基化合物如甲基铵碘化铅（MAPbI₃），在光电领域引起了一场革命。他们具有适当带隙、高吸收系数、极长载流子寿命和扩散长度等特点。目前，铁电（FE）畴被认为是解释其优异性质的一个重要因素，但学术界对此仍缺乏明确的共识和解释。

在无机钙钛矿家族中，大多数具有较小Goldschmidt公差因子的钙钛矿氧化物不是铁电材料。MAPbI₃虽然具有较小的公差因子，但与传统无机钙钛矿不同的是，它在A位点存在极性分子，该极性分子与PbI₆–八面体之间复杂的相互作用对铁电性质可能有很大影响。然而，目前还没有关于描述分子取向、极性扭曲和反铁畸变（AFD）旋转运动的演化的全面报道。

Fig. 2 The role of PbI6-octahedra rotations on reorientation of MA+ molecules from cubic to tetragonal phase.

来自比利时列日大学的Philippe Ghosez等，利用第一性原理密度泛函理论计算，系统地研究了MAPbI₃中从立方相到四方相再到正交相的铁电性质。他们发现，MAPbI₃本应该在冷却过程中出现有序–无序相变，并在居里温度T_FE时，转向铁电T-[111]相。

然而，在T=162 K时，PbI₆八面体的旋转使系统转变为Pnma正交晶格相，这个相比T-[111]相能量更低。AFD同相和异相运动的结合进一步扭曲了A位点空位，使MA⁺分子按照反极性模式沿<100>方向排列。

因此，尽管在铁弹畴处MA⁺偶极矩的重排可能具有一些极性特性，但MAPbI₃的基态并不是铁电态。这些结论只针对于MAPbI₃材料，不一定能成为类似杂化钙钛矿的一般性质。

作者所提到的方法和策略是通用的，可能有助于解决相关杂化钙钛矿的潜在铁电性质。相关论文发布于npj Computational Materials 8: 165 (2022)。

Editorial Summary

The missed ferroelectricity in MAPbI₃

Organic-inorganic halide perovskites with the chemical formula ABX₃, especially lead-based compounds such as methylammonium lead iodide (MAPbI₃), have recently revolutionized the field of optoelectronics due to their appropriate band gap, high absorption coefficient, and extremely long carrier lifetime and diffusion length. Currently, ferroelectric (FE) domains are believedto be an important factor in explaining the outstanding properties of MAPbI₃, but there is still a lack of definitive consensus and explanation in the academic community. Among the inorganic perovskite family, most perovskite oxides with a small Goldschmidt tolerance factor. Although MAPbI₃has a small tolerance factor, it is different from traditional inorganic perovskites in that it has polar molecules at the A-site, which may have a significant impact on its ferroelectric properties due to complex interactions with the PbI₆ octahedra. However, no global picture describing the evolution of molecule orientations, polar distortion and antiferrodistortive (AFD) rotation motions in all three phases has been reported yet. In this work, Philippe Ghosez et al. from the University of Liège, Belgium, systematically investigated the FE properties in MAPbI₃ from the cubic to the tetragonal and then orthorhombic phases by using first-principles density functional theory calculations. It was shown that MAPbI₃ should a priori exhibit on cooling an order-disorder transition toward the FE T-[111] phase at a given Curie temperature T_FE. However, at T = 162 K, appearance of additional PbI₆ octahedra rotations brings the system into a Pnma orthorhombic phase. This phase is significantly lower in energy than the T-[111] phase, and the combination of in-phase and out-of-phase AFD motions further distorts the A-site cavities in such a way that the MA⁺ molecules are now forced to align along <100> directions according to a well-defined antipolar pattern. The ground state of MAPbI₃ is therefore clearly not FE, although the rearrangement of MA⁺ dipoles at ferroelastic domain walls might possibly contribute to providing some polar character. This article was recently published in npj Computational Materials 8,: 165 (2022).

原文Abstract及其翻译

Missed ferroelectricity in methylammonium lead iodide (甲基铵碘化铅中丢失的铁电性)

Wen-Yi Tong, Jin-Zhu Zhao & Philippe Ghosez 

Abstract Methylammonium lead iodide, as related organometal halide perovskites, emerged recently as a particularly attractive material for photovoltaic applications. The origin of its appealing properties is sometimes assigned to its potential ferroelectric character, which remains however a topic of intense debate. Here, we rationalize from first-principles calculations how the spatial arrangement of methylammonium polar molecules is progressively constrained by the subtle interplay between their tendency to bond with the inorganic framework and the appearance of iodine octahedra rotations inherent to the perovskite structure. The disordered tetragonal phase observed at room temperature is paraelectric. We show that it should a priori become ferroelectric but that iodine octahedra rotations drive the system toward an antipolar orthorhombic ground state, making it a missed ferroelectric.

摘要 甲基铵碘化铅是一种有机金属卤化物钙钛矿材料，近年来作为光伏材料备受关注。其吸引人之处有时被认为源于其潜在的铁电特性，但这仍然处于激烈的争议之中。在本文中，我们通过第一性原理计算，解释了甲基铵极化分子的空间排列方式如何逐渐受到它们与无机框架结合的倾向性以及与钙钛矿结构固有的碘八面体旋转之间微妙的相互作用所限制的。在室温下观察到的无序四方相是顺电的。我们发现，它本应成为铁电体，但由于碘八面体的旋转，系统向反极性正交晶格基态演化，成为一个丢失的铁电体。