对称破裂：层状钙钛矿负热膨胀的原因
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ABO3型钙钛矿和层状钙钛矿在其高对称相中通常具有等量的组分，但为什么层状钙钛矿在很宽的温度范围内会发生负热膨胀（NTE），而同一化学家族的ABO3型钙钛矿却通常不会发生？有两个关键因素可决定NTE的程度：1）存在驱动收缩的软声子模式（具有负的Grüneisen参数）和2）各向异性弹性柔量使材料易于沿着特定轴线作NTE所需的变形。而来自英国帝国理工学院、华威大学和肯特大学的研究人员采用第一原理计算和对称性分析发现，实际是原子水平上的因素导致了这个差异。在层状钙钛矿中，八面体氧的分层和压缩旋转两个效应，共同导致了对称破裂，即层状钙钛矿中某些结构构型偏离了完全对称的位置，而不象普通钙钛矿的结构构型那样被牢牢固定，从而引起弹性各向异性显著增强，使其在很宽的温度范围内维持单轴NTE。这一重要成果提示，对称性和弹性张量可以用作新型NTE材料的高通量筛选和指引新材料设计的描述符。本文近期发表于npj Computational Materials 3: 44 (2017); doi:10.1038/s41524-017-0040-0; 标题与摘要如下，论文PDF文末点击阅读原文可以获取。
The origin of uniaxial negative thermal expansion in layered perovskites
(层状钙钛矿中单轴负热膨胀的起源)
Chris Ablitt, Sarah Craddock, Mark S. Senn, Arash A. Mostofi & Nicholas C. Bristowe
Why is it that ABO3 perovskites generally do not exhibit negative thermal expansion (NTE) over a wide temperature range, whereas layered perovskites of the same chemical family often do? It is generally accepted that there are two key ingredients that determine the extent of NTE: the presence of soft phonon modes that drive contraction (have negative Grüneisen parameters); and anisotropic elastic compliance that predisposes the material to the deformations required for NTE along a specific axis. This difference in thermal expansion properties is surprising since both ABO3 and layered perovskites often possess these ingredients in equal measure in their high-symmetry phases. Using first principles calculations and symmetry analysis, we show that in layered perovskites there is a significant enhancement of elastic anisotropy due to symmetry breaking that results from the combined effect of layering and condensed rotations of oxygen octahedra. This feature, unique to layered perovskites of certain symmetry, is what allows uniaxial NTE to persist over a large temperature range. This fundamental insight means that symmetry and the elastic tensor can be used as descriptors in high-throughput screening and to direct materials design.
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