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.