鐵電材料在電子和存儲器件中有許多有趣的應用，理解和調(diào)控其性質(zhì)是凝聚態(tài)物理和材料科學中一個非常有趣的課題。鐵電可以在空間反轉(zhuǎn)對稱斷破缺的材料中實現(xiàn)。在ABO₃鈣鈦礦氧化物中，沿著特定晶體方向形成具有長程有序的氧空位，將會導致BO₆八面體轉(zhuǎn)變成BO₄鏈，從而有望通過特殊的轉(zhuǎn)動產(chǎn)生極化。在缺氧鈣鈦礦（ABO_3-δ）中，格雷尼爾相的缺氧程度（δ= 1/3）要比褐磨鈦礦的?。?/span>δ= 0.5）。

Fig. 1 Structures of oxygen-deficient perovskites.

最近有實驗指出，極化與反鐵磁在R_1.2Ba_1.2Ca_0.6Fe₃O₈?(R= Gd, Tb)中可以共存，表明了格雷尼爾相中實現(xiàn)鐵電極化的可能性，但是相關的鐵電極化研究極少。

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

來自芝加哥大學普利茲克分子工程學院的Giulia Galli教授課題組，利用第一性原理預測了缺氧的RA₂Fe₃O₈鈣鈦礦中可以實現(xiàn)可調(diào)控的鐵電性。

Fig. 3 Distortion patterns in polar and antipolar phases.

他們發(fā)現(xiàn)，鐵電產(chǎn)生于八面體和四面體單元之間的協(xié)同畸變，并且較大的A位堿土金屬以及較小的R為稀土元素，可以使得極化變得最大并且讓極化翻轉(zhuǎn)的勢壘變得最小。

Fig. 4 Cooperative distortions and gear motions.

他們認為格雷尼爾相鈣鈦礦可能是一個研究對稱性破缺現(xiàn)象的平臺，有望在磁電耦合器件、非易失性存儲器件等方面產(chǎn)生廣泛的應用。該文近期發(fā)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及其翻譯

Tunable ferroelectricity in oxygen-deficient perovskites with Grenier structure (格雷尼爾結(jié)構(gòu)缺氧鈣鈦礦中的可調(diào)鐵電性)

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計算，我們預測在具有格雷尼爾結(jié)構(gòu)和氧空位有序的氧化物鈣鈦礦中可以實現(xiàn)可調(diào)控的鐵電性。具體來說，我們證明R1/3A2/3FeO2.67塊體材料（其中R是稀土離子，A是堿土陽離子）可以展現(xiàn)出極性結(jié)構(gòu)，且可以通過適當選擇R和A來調(diào)節(jié)自發(fā)極化。我們發(fā)現(xiàn)，較大的陽離子與較小的R元素結(jié)合，可以使得極化變得最大并且讓極化翻轉(zhuǎn)的勢壘變得最小。鐵電產(chǎn)生于八面體和四面體單元之間的協(xié)同畸變，其中旋轉(zhuǎn)和滑移模式的組合可以控制極化的出現(xiàn)。我們的結(jié)果表明，氧化鈣鈦礦中的極性格雷納結(jié)構(gòu)可用于研究中心對稱破缺所出現(xiàn)的各類現(xiàn)象，是一種很有前途的微電子應用材料。