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

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 Cl3–x (x = 0.40, 0.58, 0.85, 1.33, 1.95, 2.25, and 2.55) and MHyPbBr2.8I0.2 perovskites. X-ray diffraction data show that all materials feature a polar monoclinic P21 symmetry at room temperature. With the temperature increase, all MHyPbBr x Cl3–x perovskites undergo a displacive phase transition to another polar orthorhombic Pb21 m phase at T2 ≥ 318 K. The bromine rich crystals (x ≥ 1.33) exhibit an additional order–disorder phase transition to the archetypal cubic Pm3̅m phase at T1 ≥ 409 K. In contrast to MHyPbBr x Cl3–x perovskites, MHyPbBr2.8I0.2 undergoes a direct P21 to Pm3̅m phase transition. The temperature at which the cubic phase is stabilized, stability range of the Pb21 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 Pb21 m and P21, whereas in phase Pm3̅m, 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 MHyPbBr2.8I0.2. SHG studies demonstrate that doping of MHyPbCl3 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 th