Multi-Noncentrosymmetric Polar Order in 2D Hybrid Lead Chloride with Broadband Emission and High-Temperature Second-Harmonic Generation Switching

Two-dimensional lead halide perovskites represent a fascinating class of hybrid semiconductors for solar cell, light-emitting, nonlinear optical (NLO), and ferroelectric applications. A notable subset within this category is luminescent ferroelectrics, which have garnered considerable attention for their potential in integrated photoelectronic devices. In this study, we employed an organic amine halogenation strategy (also referred to as halogen engineering), which is renowned for its efficacy in inducing polar order through crystal engineering. Consequently, we synthesized a layered Ruddlesden-Popper (RP) lead chloride comprising 3-chloropropylammonium cations (CPA⁺), with the chemical formula CPA₂PbCl₄. This compound features as many as four temperature-dependent crystal phases, with phase transitions observed at <i>T</i>₁ = 353.1 K (343.9 K), <i>T</i>₂ = 211.7 K (208.6 K), and <i>T</i>₃ = 182.0 K (178.2 K) in the heating (cooling) cycles. Employing a multitechnique approach─including thermal analysis, X-ray diffraction, dielectric and pyroelectric current measurements, Raman spectroscopy, and second-harmonic generation (SHG) studies─we determined the mechanisms of the structural phase transitions. Our findings demonstrate polar order of phase <b>II</b> (space group <i>Cmc</i>2₁), phase <b>III</b> (space group <i>Pna</i>2₁), and phase <b>IV</b> (space group <i>Pca</i>2₁), while also confirming the centrosymmetric nature of phase <b>I</b> (space group <i>Cmce</i>). X-ray diffraction data revealed that the <b>I</b> to <b>II</b> PT is of a ferroelectric nature, devoid of ferroelastic strain, a conclusion further supported by pyroelectric measurements. CPA₂PbCl₄ features negative linear thermal expansion and broadband emission, which transitions to white light above 180 K. Remarkably, CPA₂PbCl₄ also demonstrates high-temperature SHG on-off switching with a high contrast ratio of 300:1 along with good switching stability, as evidenced by SHG cycling studies at heating/cooling rates ranging from 5 to 50 K/min. This SHG study also sets new standards for the field of SHG switching by providing a method to quantify the thermal responsiveness of SHG-switchable materials using the <i>t</i>_req (time requirement) parameter. Overall, our findings show that the halogenation strategy has led to the discovery of a rare example of an RP perovskite exhibiting coexistence of white-light emission, SHG <i>on-off</i> thermal bistability, ferroelectricity, and negative linear thermal expansion.