Pb₄SeBr₆: A Congruently Melting Mid‐Infrared Nonlinear Optical Material with Excellent Comprehensive Performance

Abstract Developing mid‐infrared (MIR) nonlinear optical (NLO) crystals is of current research interest, but their rational design is still faced by a great challenge because of the inherent conflict between large second harmonic generation (SHG) response and wide bandgap. To reconcile these conflicting properties, the best way is to combine multiple anions together. Up to now, various MIR NLO crystals with multiple anions are reported, such as oxychalcogenides, oxyhalides, and salt‐inclusion chalcogenide‐halides. However remarkably, the nonsalt‐inclusion chalcohalides with chalcogenide Q^2– (Q = S, Se) and halide X^– (X = F, Cl, Br) anions combined with the same central cations are still rarely reported. Herein, a lead chalcohalide Pb₄SeBr₆ is reported, which belongs to the nonsalt‐inclusion chalcohalide and is synthesized by the rational aliovalent substitution of Se^2– for Br^– anions with PbBr₂ as the template. Importantly, this compound can exhibit excellent NLO properties, including large SHG response (1.3 × AgGaS₂ (AGS)), wide IR transparent region (0.5–25 µm), and high laser damage thresholds (10 × AGS) as well as the congruently melting performance. These indicate that Pb₄SeBr₆ is a promising MIR NLO crystal and it will also provide some insights for exploring new MIR NLO crystals based on the rational partial anion substitution.