Tunable thermotropic phase transition triggering large dielectric response and superionic conduction in lead halide perovskites

Lead halide perovskites have been demonstrated as multifunctional materials. Herein, we demonstrate the tunable thermotropic phase transitions and optical bandgap, together with large dielectric response and superionic conduction through mixed halogen anion modification in a family of bromide-iodide lead halide perovskites [Et₃PrN][PbI_xBr_3−x] (Et₃PrN⁺ = triethylpropylammonium and x = 0, 1.08, 1.83, 2.50 and 3; labeled as 1-5 sequentially). All members in this family are isomorphic at room temperature, crystallizing in the monoclinic space group P2₁/n or P2₁/c, and undergo a phase transition above 440 K. DSC revealed a phase transition that is relevant to the order-to-disorder transformations of globular-shaped cations. Significantly, the lead halide perovskites exhibit superionic conduction beyond 10⁻³ S cm⁻¹ in the high-temperature (HT) phase. This study provides an efficient strategy for design of multifunctional lead halide perovskites.