论文标题
金属有机框架中分子订购的铁电性和拓扑涡流
Ferroelectricity and topological vortices from molecular ordering in metal-organic frameworks
论文作者
论文摘要
金属有机框架理解了一类带有常规结构的混合有机无机材料，$ _m $ bx $ _n $，$ a $ a $ a $ a，$ x $是有机分子，b是金属阳离子。这通常会提高结构灵活性和新功能。混合钙蛋白酶ABX $ _3 $是一个众所周知的例子。}在基于铁的钙钛矿中（DMA）Fe^{II-III}（COOH）_3，二甲基铵（DMA）分子是在六边形结构中组织的。它们在高温下是定向障碍，但在$ t = 100 $ 〜k的订单中以特殊的环形图案的命令。最近的实验和理论研究表明，尽管测得的极化较小，但在此阶段中出现了铁电偏振的出现，并且仍然对铁电的机理进行辩论。我们制定了一种Landau型理论，该理论阐明了无机晶格的电化极化，分子模式和扭曲模式之间的联系。我们发现了一种显着的铁电性机制，类似于无机六边形铁氧体和锰矿的三聚过程，但这里是由有机分子在金属有机框架中的有机分子的驱动的。我们的研究揭示了一个非常丰富的相图，并预测了拓扑结构域壁的预测，其中铁电性是由于分子有序引起的单位细胞而产生的。预测具有内部结构的宽域壁。
Metal-organic frameworks comprehend a wide class of hybrid organic-inorganic materials with general structure A$_m$BX$_n$, with $A$ and $X$ being organic molecules and B a metal cation. This often results in enhanced structural flexibility and new functionalities. Hybrid perovskites ABX$_3$ are a well-known example.} In an Iron-based perovskites, (DMA)Fe^{II-III}(COOH)_3, dimethylammonium (DMA) molecules are organized in a hexagonal structure. They are orientationally disordered at high temperatures, but order at around $T=100$~K in a peculiar toroidal pattern. Recent experimental and theoretical study suggest the appearance of ferroelectric polarization in this phase, although the measured polarization is small, and the mechanism of ferroelectricity is still debated. We formulate a Landau-type theory that clarifies the connection between the electric polarization, molecular pattern, and distortive modes of the inorganic lattice. We find a remarkable mechanism of improper ferroelectricity, analogue to the trimerization process in inorganic hexagonal ferrites and manganites, but here driven by the ordering of organic molecules in a metal-organic framework. Our study reveals an extremely rich phase diagram with the prediction of topological domain walls, where the ferroelectricity arise from tripling the unit cells due to molecular ordering. Wide domain walls with inner structure are predicted.
