Multi-Noncentrosymmetric\nPolar Order in 2D Hybrid\nLead Chloride with Broadband Emission and High-Temperature Second-Harmonic\nGeneration Switching

Two-dimensional lead halide perovskites represent a fascinating\nclass of hybrid semiconductors for solar cell, light-emitting, nonlinear\noptical (NLO), and ferroelectric applications. A notable subset within\nthis category is luminescent ferroelectrics, which have garnered considerable\nattention for their potential in integrated photoelectronic devices.\nIn this study, we employed an organic amine halogenation strategy\n(also referred to as halogen engineering), which is renowned for its\nefficacy in inducing polar order through crystal engineering. Consequently,\nwe synthesized a layered Ruddlesden–Popper (RP) lead chloride\ncomprising 3-chloropropylammonium cations (CPA<sup>+</sup>), with\nthe chemical formula CPA<sub>2</sub>PbCl<sub>4</sub>. This compound\nfeatures as many as four temperature-dependent crystal phases, with\nphase transitions observed at <i>T</i><sub>1</sub> = 353.1\nK (343.9 K), <i>T</i><sub>2</sub> = 211.7 K (208.6 K), and <i>T</i><sub>3</sub> = 182.0 K (178.2 K) in the heating (cooling)\ncycles. Employing a multitechnique approachincluding thermal\nanalysis, X-ray diffraction, dielectric and pyroelectric current measurements,\nRaman spectroscopy, and second-harmonic generation (SHG) studieswe\ndetermined the mechanisms of the structural phase transitions. Our\nfindings demonstrate polar order of phase <b>II</b> (space group <i>Cmc</i>2<sub>1</sub>), phase <b>III</b> (space group <i>Pna</i>2<sub>1</sub>), and phase <b>IV</b> (space group <i>Pca</i>2<sub>1</sub>), while also confirming the centrosymmetric\nnature of phase <b>I</b> (space group <i>Cmce</i>).\nX-ray diffraction data revealed that the <b>I</b> to <b>II</b> PT is of a ferroelectric nature, devoid of ferroelastic strain,\na conclusion further supported by pyroelectric measurements. CPA<sub>2</sub>PbCl<sub>4</sub> features negative linear thermal expansion\nand broadband emission, which transitions to white light above 180\nK. Remarkably, CPA<sub>2</sub>PbCl<sub>4</sub> also demonstrates high-temperature\nSHG on–off switching with a high contrast ratio of 300:1 along\nwith good switching stability, as evidenced by SHG cycling studies\nat heating/cooling rates ranging from 5 to 50 K/min. This SHG study\nalso sets new standards for the field of SHG switching by providing\na method to quantify the thermal responsiveness of SHG-switchable\nmaterials using the <i>t</i><sub>req</sub> (time requirement)\nparameter. Overall, our findings show that the halogenation strategy\nhas led to the discovery of a rare example of an RP perovskite exhibiting\ncoexistence of white-light emission, SHG <i>on–off</i> thermal bistability, ferroelectricity, and negative linear thermal\nexpansion.