Inorganic Chemistry · 2022 · 41 citations · 53 references
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<sup>+</sup>) 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<sub>2</sub>PbCl<sub>4</sub>. MHy<sub>2</sub>PbCl<sub>4</sub> features three temperature-dependent crystal phases, with two first-order phase transitions at T<sub>1</sub> = 338.2 K (331.8 K) and T<sub>2</sub> = 224.0 K (205.2 K) observed in the heating (cooling) modes, respectively. Observed transitions involve a transformation from high-temperature orthorhombic phase <b>I</b>, with the centrosymmetric space group <i>Pmmn</i>, through the room-temperature modulated phase <b>II</b>, with the average structure being isostructural to <b>I</b>, to the low-temperature monoclinic phase <b>III</b>, with non-centrosymmetric space group <i>P</i>2<sub>1</sub>. The intermediate phase <b>II</b> is a rare example of a modulated structure in 2D perovskites, with <i>Pmmn</i>(00γ)s00 superspace symmetry and modulation vector <b>q</b> ≅ 0.25<b>c</b>*. MHy<sub>2</sub>PbCl<sub>4</sub> beats the previous record of MHy<sub>2</sub>PbBr<sub>4</sub> 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 <b>III</b> is confirmed with second harmonic generation (SHG) measurements, and polarity is demonstrated by the pyroelectric effect. MHy<sub>2</sub>PbCl<sub>4</sub> 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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