[Methylhydrazinium] 2 PbCl 4 , a Two-Dimensional Perovskite with Polar and Modulated Phases.

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.