Pb 18 O 8 Cl 15 I 5 : A Polar Lead Mixed Oxyhalide with Unprecedented Architecture and Excellent Infrared Nonlinear Optical Properties

To develop high‐performance nonlinear optical (NLO) materials for infrared (IR) applications, we have applied a rational element‐composition design strategy and investigated the unexplored PbO–PbCl 2 –PbI 2 system. By doing so, we discovered a new polar lead mixed oxyhalide, Pb 18 O 8 Cl 15 I 5 , the first synthetic metal oxyhalide combining both Cl − and I − . Pb 18 O 8 Cl 15 I 5 reveals an unprecedented structural feature with two different dimensional types of oxocentered Pb–O units, namely, [O 4 Pb 8 ] 8+ clusters and [OPb 2 ] 2+ chains. Centimeter‐sized single crystals of Pb 18 O 8 Cl 15 I 5 have been successfully grown under ambient conditions. Remarkably, Pb 18 O 8 Cl 15 I 5 satisfies all fundamental yet rigorous criteria for high‐performance IR NLO materials, exhibiting the widest IR transparency (up to 16.0 μm) among oxide‐based crystals, strong second‐harmonic generation response (1.05×AgGaS 2 ), superior birefringence (0.086 at 636 nm), and a high laser‐induced damage threshold (8.5×AgGaS 2 ).