Department of Chemistry, University of Oxford, Inorganic Chemistry Laboratory, South Parks Road, OX1 3QR, Oxford, UK.
Department of Chemistry, University of Warwick, Gibbet Hill, CV4 7AL, Coventry, UK.
Nat Commun. 2018 Jun 18;9(1):2380. doi: 10.1038/s41467-018-04764-x.
The central goal of crystal engineering is to control material function via rational design of structure. A particularly successful realisation of this paradigm is hybrid improper ferroelectricity in layered perovskite materials, where layering and cooperative octahedral tilts combine to break inversion symmetry. However, in the parent family of inorganic ABX perovskites, symmetry prevents hybrid coupling to polar distortions. Here, we use group-theoretical analysis to uncover a profound enhancement of the number of improper ferroelectric coupling schemes available to molecular perovskites. This enhancement arises because molecular substitution diversifies the range of distortions possible. Not only do our insights rationalise the emergence of polarisation in previously studied materials, but we identify the fundamental importance of molecular degrees of freedom that are straightforwardly controlled from a synthetic viewpoint. We envisage that the crystal design principles we develop here will enable targeted synthesis of a large family of new acentric functional materials.
晶体工程的核心目标是通过合理的结构设计来控制材料的功能。这一理念的一个特别成功的实现是层状钙钛矿材料中的混合赝铁电体，其中分层和协同八面体倾斜相结合打破了反转对称。然而，在无机 ABX 钙钛矿的母体家族中，对称性阻止了与极性畸变的混合耦合。在这里，我们使用群论分析揭示了分子钙钛矿中可用的赝铁电耦合方案数量的显著增强。这种增强是因为分子取代使可能的畸变范围多样化。我们的见解不仅解释了先前研究的材料中极化的出现，而且还确定了分子自由度的基本重要性，从合成的角度来看，这些自由度是可以直接控制的。我们设想，我们在这里开发的晶体设计原则将能够有针对性地合成一大类新的非中心功能材料。
