Low-energy ferroelectric–paraelectric phase transitions of three-dimensional A(II)B(I)X3-type perovskite ferroelectrics: How to realize high phase transition temperatures

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.