A 9R-Type Hexagonal Perovskite Polytype Structure, Me₃HyPbBr₃, Exhibits Reversible Phase Transition and Fluorescent Semiconductor Properties

Recently, organic–inorganic hybrid perovskites with their unique physicochemical properties have become a vibrant frontier of research. Here, we report a novel organic–inorganic hybrid phase transition material, Me₃HyPbBr₃ (Me₃Hy⁺ = 1,1,1-trimethylhydrazine), and explore its thermal, structural, and optical properties. X-ray diffraction shows that Me₃HyPbBr₃ belongs to the R3̅m, R3̅, and R3̅m space groups in the low-temperature (LTP), intermediate-temperature (ITP), and high-temperature (HTP) phases, respectively. In addition, the compound has a unique structure, a 9R-type hexagonal perovskite polytype structure, and the inorganic framework is alternately connected by tetrahedral and octahedral [PbBr₃]⁻. Differential scanning calorimetry (DSC) tests showed that the compound underwent a reversible phase transition near 262 and 215 K. In addition, Me₃HyPbBr₃ possesses semiconducting and fluorescent properties with a band-gap value of 2.91 eV, a photoluminescence (PL) peak located at 518 nm, and a quantum yield of 17.95%, suggesting its potential for optoelectronic applications.