Thermally Induced Reversible Double Phase Transitions in an Organic–Inorganic Hybrid Iodoplumbate C4H12NPbI3 with Symmetry Breaking

A one-dimensional (1D) organic–inorganic hybrid iodoplumbate crystal (1, C4H12NPbI3, TMAPbI3) can undergo two reversible phase transitions as the temperature decreases. Its dynamic phase-transition behaviors were carefully studied by dielectric measurements, thermal analysis, and variable-temperature crystallographic studies. These results indicate that the phase transitions possess a disorder–order feature with a noncentrosymmetrical intermediate phase structure. Due to the existence of the ordered motion and reorientation of the C4H12N+ cation, 1 undergoes two phase transitions: the first one from space group P63/m at room temperature to Pm at 163 K with symmetry breaking, and the second one from space group Pm at 163 K to P61 at 142 K with partial symmetry restoration. Our results indicate that there is an existence of a transitional structure with a low symmetry space group during the disorder–order-type phase transitions, which can provide us valuable information to deeply understand the disorder–order phase transition in organic–inorganic hybrids.

一维（1D）有机-无机杂化碘铅酸盐晶体（化合物1，化学式C₄H₁₂NPbI₃，简称TMAPbI₃）可随温度降低发生两次可逆相变。针对其动态相变行为，研究人员通过介电测量（dielectric measurements）、热分析（thermal analysis）与变温晶体学研究（variable-temperature crystallographic studies）开展了细致表征。结果表明，该类相变具备无序-有序（disorder–order）特征，且存在非中心对称的中间相结构。由于C₄H₁₂N⁺阳离子存在有序运动与取向重排现象，化合物1经历两次相变：第一次为室温下空间群（space group）P6₃/m在163 K时转变为空间群Pm，伴随对称性破缺；第二次为163 K下空间群Pm在142 K时转变为空间群P6₁，伴随部分对称性恢复。本研究结果证实，在无序-有序型相变过程中存在具有低对称性空间群的过渡结构，这可为深入理解有机-无机杂化材料中的无序-有序相变机制提供宝贵的参考信息。