Polymorphic Pb14O8I12 and Pb7O4I6 oxyhalides featuring unprecedented [O8Pb14] clusters with broad IR transparency

Wide transmittance range is an essential factor for the design of infrared optical materials. Traditionally, the designs of mid-far infrared crystals have focused on oxygen-free systems of chalcogenides and pnictides. Nevertheless, the introduction of elements with large atomic numbers, such as the lone-pair cation Pb2+ and halogen anion I−, based on the oxyhalides, can broaden the infrared transmittance range. Thus, two new polymorphs of Pb(II)-containing oxyhalides, Pb14O8I12 (α-POI) and Pb7O4I6 (β-POI), are successfully synthesized via the high-temperature solution method. Interestingly, they crystallize in the same space group, \(P\bar 1\), and present unprecedented [O8Pb14] clusters from the perspective of oxo-centered [OPb4] basic units. Remarkably, POI exhibits a quite wide transparent range (0.47–16.0 µm), which enlightens the oxyhalides with prominent infrared optical properties. Additionally, the first principles calculations show that they have large birefringence (0.139 and 0.108 for α- and β-POI at 1064 nm, respectively), which makes POI promising infrared optical materials.