Effect of Dimensionality on Photoluminescence and Dielectric Properties of Imidazolium Lead Bromides.

Hybrid organic-inorganic lead halide perovskites have emerged as promising materials for various applications, including solar cells, light-emitting devices, dielectrics, and optical switches. In this work, we report the synthesis, crystal structures, and linear and nonlinear optical as well as dielectric properties of three imidazolium lead bromides, IMPbBr, IMPbBr, and IMPbBr (IM = imidazolium). We show that these compounds exhibit three distinct structure types. IMPbBr crystallizes in the 4H-hexagonal perovskite structure with face- and corner-shared PbBr octahedra (space group 6/ at 295 K), IMPbBr adopts a one-dimensional (1D) double-chain structure with edge-shared octahedra (space group 1̅ at 295 K), while IMPbBr crystallizes in the 1D single-chain structure with corner-shared PbBr octahedra (space group 1̅ at 295 K). All compounds exhibit two structural phase transitions, and the lowest temperature phases of IMPbBr and IMPbBr are noncentrosymmetric (space groups 2 at 190 K and 1 at 100 K, respectively), as confirmed by measurements of second-harmonic generation (SHG) activity. X-ray diffraction and thermal and Raman studies demonstrate that the phase transitions feature an order-disorder mechanism. The only exception is the isostructural 1̅ to 1̅ phase transition at 141 K in IMPbBr, which is of a displacive type. Dielectric studies reveal that IMPbBr is a switchable dielectric material, whereas IMPbBr is an improper ferroelectric. All compounds exhibit broadband, highly shifted Stokes emissions. Features of these emissions, , band gap and excitonic absorption, are discussed in relation to the different structures of each composition.