Three-Dimensional Methylhydrazinium Lead Halide Perovskites: Structural Changes and Effects on Dielectric, Linear, and Nonlinear Optical Properties Entailed by the Halide Tuning

High Resolution Image Download MS PowerPoint Slide Three-dimensional lead halide perovskites are promising materials for optoelectronic applications. The most famous representative comprise methylammonium (MA + ) and formamidinium (FA + ) cations, but recently, this group was enlarged by methylhydrazinium (MHy + ) analogues that crystallize in polar structures at room-temperature. Properties of three-dimensional (3D) perovskites can be tuned by mixing of molecular cations or halide anions. Here, we report synthesis and physicochemical characterization of mixed-halide MHyPbBr x Cl 3– x ( x = 0.40, 0.58, 0.85, 1.33, 1.95, 2.25, and 2.55) and MHyPbBr 2.8 I 0.2 perovskites. X-ray diffraction data show that all materials feature a polar monoclinic P 2 1 symmetry at room temperature. With the temperature increase, all MHyPbBr x Cl 3– x perovskites undergo a displacive phase transition to another polar orthorhombic Pb 2 1 m phase at T 2 ≥ 318 K. The bromine rich crystals ( x ≥ 1.33) exhibit an additional order–disorder phase transition to the archetypal cubic Pm 3̅ m phase at T 1 ≥ 409 K. In contrast to MHyPbBr x Cl 3– x perovskites, MHyPbBr 2.8 I 0.2 undergoes a direct P 2 1 to Pm 3̅ m phase transition. The temperature at which the cubic phase is stabilized, stability range of the Pb 2 1 m phase, and distortion of the lead-halide octahedra decrease with the increase of Br – content. The structural changes affect dielectric, conductivity, and optical properties. In particular, the Br-rich samples show switchable dielectric behavior near 410–420 K. Furthermore, the activation energy of Cl – ionic conductivity increases with the increase of Br – content in phases Pb 2 1 m and P 2 1, whereas in phase Pm 3̅ m, the conductivity of Br – ions increases with the increase of Cl – content. The energy band gap narrows and the photoluminescence (PL) bands exhibit red shift when going from Cl to Br and then to I. Interestingly, whereas PL of the Br-rich and Cl-rich samples is dominated by bound exciton and self-trapped exciton bands, respectively, these bands are suppressed for 2.25 ≥ x ≥ 0.85. The PL color is strongly tuned by doping and changes from greenish-blue for the Cl-rich samples to yellowish-green for MHyPbBr 2.8 I 0.2 . SHG studies demonstrate that doping of MHyPbCl 3 with Br – ions reduces the difference between SHG signal intensities of the monoclinic and orthorhombic phases, to the extent that beyond x = 1.95, the SHG response of these phases becomes essentially the same. The relative SHG efficiencies of Br–Cl mixed materials at room temperature increase with the increase in Br content.