Miao Leixin, Hasin Kishwar-E, Moradifar Parivash, Mukherjee Debangshu, Wang Ke, Cheong Sang-Wook, Nowadnick Elizabeth A, Alem Nasim
Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA, 16802, USA.
Department of Materials Science and Engineering, University of California, Merced, CA, 95343, USA.
Nat Commun. 2022 Aug 22;13(1):4927. doi: 10.1038/s41467-022-32090-w.
The layered perovskite CaMnO (CMO) is a hybrid improper ferroelectric candidate proposed for room temperature multiferroicity, which also displays negative thermal expansion behavior due to a competition between coexisting polar and nonpolar phases. However, little is known about the atomic-scale structure of the polar/nonpolar phase coexistence or the underlying physics of its formation and transition. In this work, we report the direct observation of double bilayer polar nanoregions (db-PNRs) in CaSrMnO using aberration-corrected scanning transmission electron microscopy (S/TEM). In-situ TEM heating experiments show that the db-PNRs can exist up to 650 °C. Electron energy loss spectroscopy (EELS) studies coupled with first-principles calculations demonstrate that the stabilization mechanism of the db-PNRs is directly related to an Mn oxidation state change (from 4+ to 2+), which is linked to the presence of Mn antisite defects. These findings open the door to manipulating phase coexistence and achieving exotic properties in hybrid improper ferroelectric.
层状钙钛矿CaMnO（CMO）是一种被提议用于室温多铁性的混合非本征铁电体候选材料，由于共存的极性相和非极性相之间的竞争，它还表现出负热膨胀行为。然而，关于极性/非极性相共存的原子尺度结构及其形成和转变的潜在物理机制知之甚少。在这项工作中，我们使用像差校正扫描透射电子显微镜（S/TEM）报告了在CaSrMnO中对双层极性纳米区域（db-PNRs）的直接观察。原位TEM加热实验表明，db-PNRs在高达650°C时仍可存在。电子能量损失谱（EELS）研究与第一性原理计算相结合表明，db-PNRs的稳定机制与Mn氧化态变化（从4+到2+）直接相关，这与Mn反位缺陷的存在有关。这些发现为操纵混合非本征铁电体中的相共存和实现奇异特性打开了大门。
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