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 threedimensional (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 P2 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 Pb2 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 Pm3̅ 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 P2 1 to Pm3̅ m phase transition. The temperature at which the cubic phase is stabilized, stability range of the Pb2 1 m phase, and distortion of the leadhalide 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 Pb2 1 m and P2 1 , whereas in phase Pm3̅ 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.
Hybrid organic-inorganic perovskites (HOIPs) are presently on the cutting edge of contemporary materials chemistry providing excellent and tuneable optoelectronic properties. Here we report the synthesis, crystal structures, linear optical as...
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 3 , IM 2 PbBr 4 , and IM 3 PbBr 5 (IM + = imidazolium). We show that these compounds exhibit three distinct structure types. IMPbBr 3 crystallizes in the 4H-hexagonal perovskite structure with face- and corner-shared PbBr 6 octahedra (space group P 6 3 / mmc at 295 K), IM 2 PbBr 4 adopts a one-dimensional (1D) double-chain structure with edge-shared octahedra (space group P 1̅ at 295 K), while IM 3 PbBr 5 crystallizes in the 1D single-chain structure with corner-shared PbBr 6 octahedra (space group P 1̅ at 295 K). All compounds exhibit two structural phase transitions, and the lowest temperature phases of IMPbBr 3 and IM 3 PbBr 5 are noncentrosymmetric (space groups Pna 2 1 at 190 K and P 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 P 1̅ to P 1̅ phase transition at 141 K in IM 2 PbBr 4 , which is of a displacive type. Dielectric studies reveal that IMPbBr 3 is a switchable dielectric material, whereas IM 3 PbBr 5 is an improper ferroelectric. All compounds exhibit broadband, highly shifted Stokes emissions. Features of these emissions, i.e. , band gap and excitonic absorption, are discussed in relation to the different structures of each composition.
Two-dimensional (2D) lead halide perovskites are a family of materials at the heart of solar cell, light-emitting diode, and photodetector technologies. This perspective leads to a number of synthetic efforts toward materials of this class, including those with prescribed polar architectures. The methylhydrazinium (MHy + ) cation was recently presumed to have an unusual capacity to generate non-centrosymmetric perovskite phases, despite its intrinsically nonchiral structure. Here, we witness this effect once again in the case of the Ruddlesden–Popper perovskite phase of formula MHy 2 PbCl 4 . MHy 2 PbCl 4 features three temperature-dependent crystal phases, with two first-order phase transitions at T 1 = 338.2 K (331.8 K) and T 2 = 224.0 K (205.2 K) observed in the heating (cooling) modes, respectively. Observed transitions involve a transformation from high-temperature orthorhombic phase I , with the centrosymmetric space group Pmmn , through the room-temperature modulated phase II , with the average structure being isostructural to I , to the low-temperature monoclinic phase III , with non-centrosymmetric space group P 2 1 . The intermediate phase II is a rare example of a modulated structure in 2D perovskites, with Pmmn (00γ)s00 superspace symmetry and modulation vector q ≅ 0.25 c *. MHy 2 PbCl 4 beats the previous record of MHy 2 PbBr 4 in terms of the shortest inorganic interlayer distance in 2D perovskites (8.79 Å at 350 K vs 8.66 Å at 295 K, respectively). The characteristics of phase transitions are explored with differential scanning calorimetry, dielectric, and Raman spectroscopies. The non-centrosymmetry of phase III is confirmed with second harmonic generation (SHG) measurements, and polarity is demonstrated by the pyroelectric effect. MHy 2 PbCl 4 also exhibits thermochromism, with the photoluminescence (PL) color changing from purplish-blue at 80 K to bluish-green at 230 K. The demonstration of polar characteristics for one more member of the methylhydrazinium perovskites settles a debate about whether this approach can present value for the crystal engineering of acentric solids similar to that which was recently adopted by a so-called fluorine substitution effect.
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