反极性扭曲：铁电相变随温度变化的动力学

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在具有反极性阳离子运动的材料中，不仅其基础理论令人生趣，而且技术应用也前景可期，因而受到广泛关注。但是针对这类复杂材料的研究大多集中于静态特性或布里渊区的声子，而反极性阳离子扭曲的一些重要的动力学行为，如声子振动频率的温度依赖关系，尽管可能具有非凡的动力学特性，却少有探讨。美国阿肯色大学和俄罗斯南联邦大学的Kinnary Patel等，对在静水压下BiFeO3的反极化阳离子扭曲（相比于铁电极化的阳离子扭曲）的动力学进行了原子模拟。他们首先预测了这种材料在降温时的相变过程：高温下呈现立方结构的顺电相，随着温度的降低而进入到具有氧八面体同相倾斜且长程有序的中间相，最后呈现氧八面体同相和反相倾斜共存，并具有反极性阳离子运动的Pnma态。研究发现，在顺电相中，所有的反极性阳离子都具有很高的振动频率，且这些振动模式并不依赖于温度。不仅如此，在中间相和Pnma相中，反极性阳离子振动模的数量有所增加，而这些振动模中有一部分已开始软化。研究结果可方便地拓展到类似材料在相变过程中反极性阳离子运动的动力学行为。该结果近期发表于npj ComputationalMaterials 3:34 (2017); doi:10.1038/s41524-017-0033-z; 标题与摘要如下，论文PDF文末点击阅读原文可以获取。

Dynamics ofantipolar distortions （反极性扭曲动力学）

Kinnary Patel, Sergey Prosandeev & Laurent Bellaiche

Materials possessing antipolar cationmotions are currently receiving a lot of attention because they are fundamentally intriguing while being technologically promising. Most studies devoted to these complex materials have focused on their static properties or on their zone-center phonons. As a result, some important dynamics of antipolar cation distortions, such as the temperature behavior of their phonon frequencies, have been much less investigated, despite the possibility to exhibit unusual features. Here, we report the results and analysis of atomistic simulations revealing and explaining such dynamics for BiFeO3 bulks being subject to hydrostatic pressure. It is first predicted that cooling such material yields the following phase transition sequence: the cubic paraelectric  state at high temperature, followed by an intermediate phase possessing long-range-ordered in-phase oxygen octahedral tiltings, and then the Pnma state that is known to possess antipolar cation motions in addition to in-phase and antiphase oxygen octahedral tiltings. Antipolar cation modes are found to all have high phonon frequencies that are independent of temperature in the paraelectric phase. On the other hand and in addition to antipolar cation modes increasing in number, some phonons possessing antipolar cation character are rather soft in the intermediate and Pnma states. Analysis of our data combined with the development of a simple model reveals that such features originate from a dynamical mixing between pure antipolar cation phonons and fluctuations of oxygen octahedral tiltings, as a result of a specific trilinear energetic coupling. The developed model can also be easily applied to predict dynamics of antipolar cation motions for other possible structural paths bringing  to Pnma states.

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