Nature子刊：铁电材料对应力出现异常反应，颠覆传统理论(双语)

材料牛注：铁电材料受力越大，电极极性就越强？当大部分人还沉浸在这样的惯性思维里时，美国西北大学的科学家已经发现了部分铁电材料不同寻常的特性：当铁电氧化物所受应力增加到一定程度，其电极或可消失不见！

Up until recently, researchers thought they had the behaviors of ferroelectric materials mostly figured out.

就在不久之前，科学家们还以为有关铁电材料的性能，他们已经研究得差不多了。

"The conventional wisdom is that you can put almost any material under mechanical stress, and provided the stress is coherently maintained, the material will become ferroelectric or exhibit an electrical polarization," said James Rondinelli, assistant professor of materials science and engineering at Northwestern University's McCormick School of Engineering. "If you apply similar stresses to a compound that's already ferroelectric, then its polarization increases."

“传统观点认为，对几乎所有材料施加机械应力，且在持续加载的条件下，材料会拥有铁电特性或表现出电极化现象，”美国西北大学助理教授James Rondinelli说道，“如果你向已经具有铁电特性的化合物施加同样的应力，其极化强度就会增加。”

Rondinelli and his team, however, have made a theoretical discovery that flips this widely accepted fact on its head. They found that when a unique class of ferroelectric oxides are stretched or compressed, the polarization does not simply increase as expected. Instead, it goes away completely. "Based on everything we have known for the past two decades," Rondinelli said, "this is completely unexpected."

然而，Rondinelli及其团队却发现了一项能够颠覆这一传统观点的理论：当某一种特定的铁电氧化物受拉或是受压时，其极化强度并非如预期那样简单地增加，而是消失得无影无踪。Rondinelli说：“回溯过去二十年里的科研发现，这种现象前所未有。”

Supported by the National Science Foundation, the research is described in the June 13 issue of Nature Materials. Xue-Zeng Lu, a PhD student in Rondinelli's laboratory, served as the paper's first author.

这项研究受美国国家科学基金会资助，并将其成果发表在6月13号的 Nature Materials 上。Rondinelli研究室的博士生Xue-Zeng Lu是该文的第一作者。

Ferroelectrics are found everywhere: in smart phones, watches, and computers. Because they are so technologically useful, researchers have long been interested in creating new or improved ferroelectric materials — especially in two-dimensional geometries as thin films where they are readily integrated into electronic devices. Ferroelectricity is a property that occurs when a material exhibits a spontaneous electric polarization, which arises from is a shift of positive and negative charges in opposite directions.

铁电材料随处可见，在智能手机、手表和电脑里都能觅其踪影。铁电材料的技术应用如此之广，以至于一直以来，科学家们都致力于这种材料的研发和改进，尤其作为薄膜集成于电子设备中。铁电特性即正负电荷突然转向时，材料呈现出自发电极的特性。

When strain is applied to the class of oxides called layered perovskites grown as a thin film, they initially react the same way as other ferroelectrics. Their polarization increases. But if further strain is applied, the polarization completely turns off.

当我们对一种名为层状钙钛矿的薄膜氧化物材料施以应力时，最初它们表现出与其他铁电材料相同的特性，即极化程度增加，但当应力进一步增大，其极化强度竟会荡然无存。

Layered perovskites have recently seen a resurgence of attention because they host functional physical properties like high-temperature super conductivity and support electrochemical or photocatalytic energy conversion processes. Their structures are also much more defect tolerant. Rondinelli's discovery adds a new level of interest to these popular materials.

层状钙钛矿材料具备一定功能物理特性，例如高温超导、能够进行电化学或光催化能量转换过程等，所以最近，这种材料又重新受到了关注。该材料的结构特点也使其允许存在更多的晶体缺陷。Rondinelli的科学发现大大提高了人们对这些材料的研究热度。

"You can't strain the material too much because it might lose its functionality," Rondinelli said. "But if you operate near where the polarization turns on and off, you really have a switch. If you're monitoring the polarization for a logic device or memory element, you can apply a small electric field to traverse this boundary and simultaneously read and write the on-and-off state."

Rondinelli说：“如果对这种材料施加太大的拉力，它可能会丧失功能特性。但我们在恰能使其极性消长的范围内对它施以应力，就能把它变为开关装置。如果这一极性的有无能够被逻辑装置或是记忆元件测得，我们就可以用一微弱电场来改变其极性，同时获知极性的有无状态。”

Rondinelli's team made this discovery using a theoretical materials tools and quantum mechanical simulations and is now working with experimental collaborators to validate the finding in the laboratory. Another next step is to better understand how this new functionality could help or hinder ferroelectric applications.

Rondinelli团队的科学发现也要归功于量子力学模拟这一材料理论研究工具，这一成果现由其合作者进行实验验证。接下来，他们将对这一新功能进行进一步探索，研究其对铁电装置的助益或阻碍机理。

In the meantime, Rondinelli said researchers will now need to be careful when applying mechanical stress to layered perovskite ferroelectrics. Applying too much strain could have unintended consequences.

与此同时，Rondinelli表示，研究人员在对层状钙钛矿材料施加机械应力时应当谨慎。应力过大会导致不可预测的后果。

"This finding motivates us to recalibrate our intuition regarding what interactions are expected between mechanical forces and dielectric properties," Rondinelli said. "It requires us to think more carefully, and I suspect there is much more to learn."

“这一发现重塑了我们关于机械应力和介电性能关系的认知” Rondinelli说，“它敦促我们谨慎思考，这里面一定还大有文章。”

新闻链接：Ferroelectric materials react unexpectedly to strain

文献链接：Epitaxial-strain-induced polar-to-nonpolar transitions in layered oxides

本文由编辑部王宇提供素材，张文扬编译，赵建伟审核