Pb18O8Cl15I5: 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 ).