Young Joshua, Stroppa Alessandro, Picozzi Silvia, Rondinelli James M
Department of Materials Science and Engineering, Drexel University, Philadelphia PA, 19104, USA.
Dalton Trans. 2015 Jun 21;44(23):10644-53. doi: 10.1039/c4dt03521f. Epub 2015 Jan 12.
Recent experimental and theoretical work has shown that the double perovskite NaLaMnWO(6) exhibits antiferromagnetic ordering owing to the Mn d states, and computational studies further predict it to exhibit a spontaneous electric polarization due to an improper mechanism for ferroelectricity [King et al., Phys. Rev. B: Condens. Matter, 2009, 79, 224428; Fukushima et al., Phys. Chem. Chem. Phys., 2011, 13, 12186], which make it a candidate multiferroic material. Using first-principles density functional calculations, we investigate nine isostructural and isovalent AA'MnWO(6) double perovskites (A = Na, K, and Rb; A' = La, Nd, and Y) with the aim of articulating crystal-chemistry guidelines describing how to enhance the magnitude of the electric polarization through chemical substitution of the A-site while retaining long-range magnetic order. We find that the electric polarization can be enhanced by up to 150% in compounds which maximize the difference in the ionic size of the A and A' cations. By examining the tolerance factors, bond valences, and structural distortions (described by symmetry-adapted modes) of the nine compounds, we identify the atomic scale features that are strongly correlated with the ionic and electronic contributions to the electric polarization. We also find that each compound exhibits a purely electronic remnant polarization, even in the absence of a displacive polar mode. The analysis and design strategies presented here can be further extended to additional members of this family (B = Fe, Co, etc.), and the improper ferroelectric nature of the mechanism allows for the decoupling of magnetic and ferroelectric properties and the targeted design of novel multiferroics.
近期的实验和理论研究表明，双钙钛矿NaLaMnWO(6)由于Mn的d态而呈现反铁磁有序，并且计算研究进一步预测，由于一种非本征铁电机制，它会表现出自发电极化[金等人，《物理评论B：凝聚态物质》，2009年，79卷，224428页；福岛等人，《物理化学化学物理》，2011年，13卷，12186页]，这使其成为一种候选的多铁性材料。我们使用第一性原理密度泛函计算，研究了九种同结构且等价的AA'MnWO(6)双钙钛矿（A = Na、K和Rb；A' = La、Nd和Y），目的是阐明晶体化学指导原则，描述如何通过A位的化学取代来增强极化强度，同时保持长程磁有序。我们发现，在使A和A'阳离子的离子尺寸差异最大化的化合物中，极化强度可增强高达150%。通过研究这九种化合物的容差因子、键价和结构畸变（由对称适配模式描述），我们确定了与极化的离子和电子贡献密切相关的原子尺度特征。我们还发现，即使在没有位移极化模式的情况下，每种化合物也都表现出纯电子剩余极化。这里提出的分析和设计策略可以进一步扩展到该家族的其他成员（B = Fe、Co等），并且该机制的非本征铁电性质允许磁和铁电性质的解耦以及新型多铁性材料的定向设计。
