Pb18 O8 Cl15 I5 : 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-PbCl2 -PbI2 system. By doing so, we discovered a new polar lead mixed oxyhalide, Pb18 O8 Cl15 I5 , the first synthetic metal oxyhalide combining both Cl- and I- . Pb18 O8 Cl15 I5 reveals an unprecedented structural feature with two different dimensional types of oxocentered Pb-O units, namely, [O4 Pb8 ]8+ clusters and [OPb2 ]2+ chains. Centimeter-sized single crystals of Pb18 O8 Cl15 I5 have been successfully grown under ambient conditions. Remarkably, Pb18 O8 Cl15 I5 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×AgGaS2 ), superior birefringence (0.086 at 636 nm), and a high laser-induced damage threshold (8.5×AgGaS2 ).