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${}_2$). (© 2020 Wiley‐VCH GmbH)