Phase transitions in MHy 2 PbCl 4 , a two-dimensional perovskite with polar and modulated phases

Katarzyna Fedoruk ³ Dawid Drozdowski ¹ Jan K. Zareba ⁴ Miroslaw Maczka ² Dagmara Stefanska ² Anna Gagor ¹ Adam Sieradzki ³

¹Structure Research, Institute of Low Temperatures and Structure Research PAS, Wroclaw, Poland
²Optical Spectroscopy, Institute of Low Temperatures and Structure Research PAS, Wroclaw, Poland
³Department of Experimental Physics, Wrocław University of Science and Technology, Wroclaw, Poland
⁴Advanced Materials Engineering and Modeling Group, Wrocław University of Science and Technology,, Wroclaw, Poland

The methylhydrazine (MHy⁺) cation has the remarkable ability to generate non-concentric perovskite phases, despite its intrinsically non-chiral structure. MHy+ is small enough to maintain 3D alignment (see MHyPbBr₃ and MHyPbCl_³ [1, 2]), and simultaneously sufficiently large to separate the 2D perovskite layers (MHy₂PbI₄ [3], MHy₂PbBr₄ [4]). Continuation of the halide substitution approach in the MHy₂PbX₄ (X = halide) 2D HOIPs has led to development of another representative with the acentric phases, i.e., MHy₂PbCl₄.

MHy₂PbCl₄ features three temperature-dependent crystal phases, with two first-order phase transitions at T₁= 338.2 K (331.8 K) and T₂= 224.0 K (205.2 K). The transitions comprise symmetry lowering from the high-temperature orthorhombic Pmmn phase I, through the room-temperature modulated phase II, and finally to the low-temperature monoclinic phase III, with non-centrosymmetric space group P2₁. The non-centrosymmetry of phase III is confirmed with second harmonic generation (SHG), whereas polarity is demonstrated by the pyroelectric effect. 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 ≅ 0.25c*. While the HT phase, is isostructural to the bromine and iodine analogues, both LT phases are unique for MHy₂PbCl₄. MHy₂PbCl₄exhibits thermochromism, with the photoluminescence (PL) color changing from purplish-blue at 80 K to bluish-green at 230 K.

Here we use single-crystal x-ray diffraction, differential scanning calorimetry, dielectric, and Raman spectroscopies to characterize MHy₂PbCl₄ and understand the structure-property relations.

References

[1] M.Mączka et al., Chemistry of Materials, 2020 32 (4), 1667-1673

[2] M.Mączka et al., Chemistry of Materials, 2020 32 (9), 4072-4082

[3] M.Mączka, et al., Chemistry of Materials, 2019 31 (20), 8563-8575

[4] M. Mączka, et al., Chemistry of Materials 2021 33 (7), 2331-2342

Phase transitions in MHy2PbCl4, a two-dimensional perovskite with polar and modulated phases

Phase transitions in MHy₂PbCl₄, a two-dimensional perovskite with polar and modulated phases