Organohalide lead perovskites are widely used in memory components, sensors, and photocells, owing to their excellent (opto)electronic or ferroelectric properties, especially light weight, and easy processing compared to pure inorganic perovskites. For their commercialization, the exploration of environmentally friendly lead-free perovskites has become a hot issue. Recently, a class of metal-free three-dimensional (3D) halide perovskites has been synthesized, and these perovskites exhibit superior ferroelectricity. With reference to the experiment-synthesized (MDABCO)(NH₄)I₃ (MDABCO = N-methyl-N′-diazabicyclo[2.2.2]octonium), two new perovskites (MDABCO)KI₃ and (MDABCO)RbI₃ are designed through replacing NH₄⁺ by metal ions. By using the first-principles calculations, we find that these (MDABCO)BI₃ (B = NH₄⁺, K⁺, and Rb⁺) perovskites all have a similar phase transition mechanism with ignorable barriers but prominent ferroelectricity (large spontaneous polarizations and high phase transition temperatures). Our design strategy can have important implications for the development of novel 3D perovskite ferroelectrics following this specific low-energy phase transition mechanism.

中文翻译：

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三维A（II）B（I）X3型钙钛矿铁电体的低能铁电-顺电相变：如何实现高相变温度

有机卤化物铅钙钛矿具有优异的（光电）电子或铁电性能，尤其是重量轻，并且与纯无机钙钛矿相比易于加工，因此被广泛用于存储组件，传感器和光电池。对于其商业化而言，探索环保的无铅钙钛矿已经成为热门问题。近来，已经合成了一类不含金属的三维（3D）卤化物钙钛矿，并且这些钙钛矿表现出优异的铁电性。参照实验合成的（MDABCO）（NH ₄）I ₃（MDABCO = N-甲基-N'-二氮杂双环[2.2.2] oc），两个新的钙钛矿（MDABCO）KI ₃和（MDABCO）RbI ₃通过用金属离子代替NH ₄ ⁺来设计。通过第一性原理计算，我们发现这些（MDABCO）BI ₃（B = NH ₄ ⁺，K ⁺和Rb ⁺）钙钛矿都具有相似的相变机制，具有可忽略的势垒，但铁电性显着（自发极化和相变温度高）。我们的设计策略对于遵循这种特定的低能量相变机理的新型3D钙钛矿铁电材料的开发可能具有重要的意义。