近年來，化學式為ABX₃的有機金屬鹵化物鈣鈦礦，特別是鉛基化合物如甲基銨碘化鉛（MAPbI₃），在光電領域引起了一場革命。他們具有適當帶隙、高吸收系數(shù)、極長載流子壽命和擴散長度等特點。目前，鐵電（FE）疇被認為是解釋其優(yōu)異性質(zhì)的一個重要因素，但學術(shù)界對此仍缺乏明確的共識和解釋。

在無機鈣鈦礦家族中，大多數(shù)具有較小Goldschmidt公差因子的鈣鈦礦氧化物不是鐵電材料。MAPbI₃雖然具有較小的公差因子，但與傳統(tǒng)無機鈣鈦礦不同的是，它在A位點存在極性分子，該極性分子與PbI₆–八面體之間復雜的相互作用對鐵電性質(zhì)可能有很大影響。然而，目前還沒有關(guān)于描述分子取向、極性扭曲和反鐵畸變（AFD）旋轉(zhuǎn)運動的演化的全面報道。

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

來自比利時列日大學的Philippe Ghosez等，利用第一性原理密度泛函理論計算，系統(tǒng)地研究了MAPbI₃中從立方相到四方相再到正交相的鐵電性質(zhì)。他們發(fā)現(xiàn)，MAPbI₃本應該在冷卻過程中出現(xiàn)有序–無序相變，并在居里溫度T_FE時，轉(zhuǎn)向鐵電T-[111]相。

然而，在T=162 K時，PbI₆八面體的旋轉(zhuǎn)使系統(tǒng)轉(zhuǎn)變?yōu)?/span>Pnma正交晶格相，這個相比T-[111]相能量更低。AFD同相和異相運動的結(jié)合進一步扭曲了A位點空位，使MA⁺分子按照反極性模式沿<100>方向排列。

因此，盡管在鐵彈疇處MA⁺偶極矩的重排可能具有一些極性特性，但MAPbI₃的基態(tài)并不是鐵電態(tài)。這些結(jié)論只針對于MAPbI₃材料，不一定能成為類似雜化鈣鈦礦的一般性質(zhì)。

作者所提到的方法和策略是通用的，可能有助于解決相關(guān)雜化鈣鈦礦的潛在鐵電性質(zhì)。相關(guān)論文發(fā)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及其翻譯

Missed ferroelectricity in methylammonium lead iodide (甲基銨碘化鉛中丟失的鐵電性)

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?甲基銨碘化鉛是一種有機金屬鹵化物鈣鈦礦材料，近年來作為光伏材料備受關(guān)注。其吸引人之處有時被認為源于其潛在的鐵電特性，但這仍然處于激烈的爭議之中。在本文中，我們通過第一性原理計算，解釋了甲基銨極化分子的空間排列方式如何逐漸受到它們與無機框架結(jié)合的傾向性以及與鈣鈦礦結(jié)構(gòu)固有的碘八面體旋轉(zhuǎn)之間微妙的相互作用所限制的。在室溫下觀察到的無序四方相是順電的。我們發(fā)現(xiàn)，它本應成為鐵電體，但由于碘八面體的旋轉(zhuǎn)，系統(tǒng)向反極性正交晶格基態(tài)演化，成為一個丟失的鐵電體。