特殊结构钙钛矿：可控铁电性

铁电材料在电子和存储器件中有许多有趣的应用，理解和调控其性质是凝聚态物理和材料科学中一个非常有趣的课题。铁电可以在空间反转对称断破缺的材料中实现。在ABO₃钙钛矿氧化物中，沿着特定晶体方向形成具有长程有序的氧空位，将会导致BO₆八面体转变成BO₄链，从而有望通过特殊的转动产生极化。在缺氧钙钛矿（ABO_3-δ）中，格雷尼尔相的缺氧程度（δ= 1/3）要比褐磨钛矿的小（δ= 0.5）。

Fig. 1 Structures of oxygen-deficient perovskites.

最近有实验指出，极化与反铁磁在R_1.2Ba_1.2Ca_0.6Fe₃O₈ (R= Gd, Tb)中可以共存，表明了格雷尼尔相中实现铁电极化的可能性，但是相关的铁电极化研究极少。

Fig. 2 Arrangements of tetrahedral chains and corresponding distortion patterns in oxygen-deficient phases.

来自芝加哥大学普利兹克分子工程学院的Giulia Galli教授课题组，利用第一性原理预测了缺氧的RA₂Fe₃O₈钙钛矿中可以实现可调控的铁电性。

Fig. 3 Distortion patterns in polar and antipolar phases.

他们发现，铁电产生于八面体和四面体单元之间的协同畸变，并且较大的A位碱土金属以及较小的R为稀土元素，可以使得极化变得最大并且让极化翻转的势垒变得最小。

Fig. 4 Cooperative distortions and gear motions.

他们认为格雷尼尔相钙钛矿可能是一个研究对称性破缺现象的平台，有望在磁电耦合器件、非易失性存储器件等方面产生广泛的应用。该文近期发布于npj Computational Materials 9: 218 (2023)。

Fig. 5 Polarization and switching barrier in Nd1/3Ca2/3FeO2.67.

Editorial Summary

Ferroelectric materials have found many interesting applications in electronic and memory devices, and understanding and engineering their properties is a topic of great interest in condensed matter physics and materials science. Ferroelectricity can be realized in materials with broken spatial inversion symmetry. The formation of long-ordered oxygen vacancies in ABO₃ perovskite oxides along specific crystal directions will cause the transformation of BO₆octahedral units into BO₄ tetrahedral chains, which is expected to generate polarization through special twist. Among oxygen-deficient perovskites (ABO_3−δ), δ is much smaller in the so-called Grenier phase (δ=1/3) than brownmillerites (δ=0.5). Recent experiments showed a spontaneous polarization coexisting with antiferromagnetism in R_1.2Ba_1.2Ca_0.6Fe₃O₈ (R= Gd, Tb), pointing at the possibility of realizing stable polar Grenier phases. However, the relevant studies about the polarization of Grenier phases are quite rare. 

Fig. 6 Cation-dependent energy differences between distortion patterns in Grenier structure.

A group led by Prof. Giulia Galli from the Pritzker School of Molecular Engineering, University of Chicago, used using the first principles calculations to predict tunable ferroelectricity in oxygen-deficient RA₂Fe₃O₈ perovskites with ordered oxygen vacancies,. They found that ferroelectricity can arise from cooperative distortions of octahedral and tetrahedral units, and larger cations combined with small R elements would lead to a maximum in the polarization and to a minimum in the FE switching barriers. 

They suggested that Grenier phase perovskites may be a promising platform to study the symmetry breaking phenomenon, which is expected to have a wide range of applications in magnetoelectric and non-volatile devices. This article was recently published in npj Computational Materials 9: 192 (2023).

原文Abstract及其翻译

Tunable ferroelectricity in oxygen-deficient perovskites with Grenier structure (格雷尼尔结构缺氧钙钛矿中的可调铁电性)

Yongjin Shin & Giulia Galli 

Abstract Using first-principles calculations, we predict that tunable ferroelectricity can be realized in oxide perovskites with the Grenier structure and ordered oxygen vacancies. Specifically, we show that R1/3A2/3FeO2.67 solids (where R is a rare-earth ion and A an alkaline-earth cation) exhibit polar phases, with a spontaneous polarization tunable by an appropriate choice of R and A. We find that larger cations combined with small R elements lead to a maximum in the polarization and to a minimum in the energy barriers required to switch the sign of the polarization. Ferroelectricity arises from cooperative distortions of octahedral and tetrahedral units, where a combination of rotational and sliding modes controls the emergence of polarization within three-dimensional connected layers. Our results indicate that polar Grenier phases of oxide perovskites are promising materials for microelectronic applications and, in general, for the study of phenomena emerging from breaking inversion symmetry in solids.

摘要利用第一性原理的计算，我们预测在具有格雷尼尔结构和氧空位有序的氧化物钙钛矿中可以实现可调控的铁电性。具体来说，我们证明R1/3A2/3FeO2.67块体材料（其中R是稀土离子，A是碱土阳离子）可以展现出极性结构，且可以通过适当选择R和A来调节自发极化。我们发现，较大的阳离子与较小的R元素结合，可以使得极化变得最大并且让极化翻转的势垒变得最小。铁电产生于八面体和四面体单元之间的协同畸变，其中旋转和滑移模式的组合可以控制极化的出现。我们的结果表明，氧化钙钛矿中的极性格雷纳结构可用于研究中心对称破缺所出现的各类现象，是一种很有前途的微电子应用材料。