Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) have recently gained tremendous interest because of their unique features in contrast to three-dimensional counterparts and traditional 2D materials. However, although some 2D HOIP ferroelectrics have been achieved, the issue of toxic Pb and uniaxial nature impede their further application. Herein, for the first time, we report a lead-free 2D HOIP multiaxial ferroelectric, [3,3-difluorocyclobutylammonium]₂CuCl₄ (1), which shows four ferroelectric axes and eight equivalent polarization directions, more than those of the other 2D HOIP ferroelectrics and even the inorganic perovskite ferroelectric BaTiO₃ (three ferroelectric axes and six equivalent polarization directions). 1 also features a high Curie temperature of 380 K and exhibits remarkable thermochromism of color change from green-yellow to dark brown. To our knowledge, 1 is the first multiaxial lead-free 2D HOIP ferroelectric. This work sheds light on the exploration of better lead-free 2D HOIP ferroelectrics.

中文翻译：

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多轴无铅二维有机-无机钙钛矿铁电体

二维 (2D) 杂化有机-无机钙钛矿 (HOIP) 最近因其与三维对应物和传统二维材料相比的独特特征而引起了极大的兴趣。然而，尽管已经实现了一些 2D HOIP 铁电体，但有毒铅和单轴性质的问题阻碍了它们的进一步应用。在此，我们首次报道了一种无铅 2D HOIP 多轴铁电体，[3,3-二氟环丁基铵] ₂ CuCl ₄ ( 1 )，它显示出四个铁电轴和八个等效极化方向，比其他 2D HOIP 铁电体，甚至无机钙钛矿铁电体 BaTiO ₃（三个铁电轴和六个等效极化方向）。1还具有 380 K 的高居里温度，并表现出显着的从绿黄色到深棕色颜色变化的热致变色现象。据我们所知，1是第一个多轴无铅 2D HOIP 铁电体。这项工作阐明了对更好的无铅 2D HOIP 铁电体的探索。