npj：钙钛矿—氧硫阴离子顺式排列引起铁电性

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钙钛矿化合物（通式为ABX₃）是一类功能丰富的材料，在催化、电子设备、传感器和能量存储等领域具有广泛应用。通常，钙钛矿的性质对微小的结构变化十分敏感，例如：金属阴离子八面体的旋转、变形和倾斜。在氧硫化钙钛矿中（或更广泛地在异质阴离子钙钛矿中），B-位八面体配位受两个或多个阴离子控制。原则上，给定金属离子周围的不同阴离子构型可能导致不同级别的晶体场强度、键合特性和局部极化，进而决定了基态的结构和功能。尽管混合阴离子钙钛矿具有丰富的潜在结构-性质关系，但是，对混合硫族化物钙钛矿中阴离子顺序的研究仍然很少。

来自美国洛斯阿拉莫斯国家实验室的Pilania博士及其合作者，利用第一性原理研究了AB(O_0.5S_0.5)₃钙钛矿中A = Ca、Sr、Ba和B = Ti、Zr、Hf的阴离子有序。他们首先分析了20个原子构成的AB(O_0.5S_0.5)₃超胞中所有246个独特对称性构型，并提炼出控制其相对能量和带隙变化的各种因素。证明了阴离子排序始终是围绕HfO₃S₃八面体的阴离子全顺式排列。这种有序趋势的起源可追溯到电子、弹性和静电作用的综合稳定效应。这些定性概念也可以使用最新的机器学习模型进行量化。他们进一步研究了所确定的有序排列的相对稳定性，该相对稳定性是A和B位点化学的函数。他们还探测了材料的电子结构和功能性依赖于化学反应的趋势。值得指出的是，他们发现确定的基态阴离子有序打破了反演对称性，从而创建了一个宏观极化大于30 μC/cm²的氧硫化物铁电体家族，对电子材料应用具有重要的前景。

该文近期发表于npj Computational Materials 6: 71 (2020)，英文标题与摘要如下，点击左下角“阅读原文”可以自由获取论文PDF。

Anion order in oxysulfide perovskites: origins and implications 

Ghanshyam Pilania, Ayana Ghosh, Steven T. Hartman, Rohan Mishra, Christopher R. Stanek & Blas P. Uberuaga 

Heteroanionic oxysulfide perovskite compounds represent an emerging class of new materials allowing for a wide range of tunability in the electronic structure that could lead to a diverse spectrum of novel and improved functionalities. Unlike cation ordered double perovskites—where the origins and design rules of various experimentally observed cation orderings are well known and understood—anion ordering in heteroanionic perovskites remains a largely uncharted territory. In this contribution, we present and discuss insights that have emerged from our first-principles-based electronic structure analysis of a prototypical anion-ordered SrHf(O_0.5S_0.5)₃  oxysulfide chemistry, studied in all possible anion configurations allowed within a finite size supercell. We demonstrate that the preferred anion ordering is always an all-cis arrangement of anions around an HfO₃S₃ octahedron. As a general finding beyond the specific chemistry, the origins of this ordering tendency are traced back to a combined stabilization effect stemming from electronic, elastic, and electrostatic contributions. These qualitative notions are also quantified using state-of-the-art machine learning models. We further study the relative stability of the identified ordering as a function of A (Ca, Sr, Ba) and B (Ti, Zr, Hf) site chemistries and probe chemistry-dependent trends in the electronic structure and functionality of the material. Most remarkably, we find that the identified ground-state anion ordering breaks the inversion symmetry to create a family of oxysulfide ferroelectrics with a macroscopic polarization >30 μC/cm², exhibiting a significant promise for electronic materials applications. 

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