Tuning birefringence in alkaline-earth metal oxyhalides through diverse mixed-cation coordination architectures

A series of new alkali-earth metal complex oxyhalides, Ba₅Pb₄O₄Br₁₀ (4PbO·5BaBr₂), Ba₂CdSe₂O₆Cl₂ (BaCdSe₂O₆·BaCl₂) and BaCdSeO₃Br₂ (CdSeO₃·BaBr₂) have been synthesized by a high-temperature solution method. These compounds exhibit layered structures composed of polyhedra in various configurations, along with distinct symmetry, bonding modes, and cation coordination orientations. Further investigations indicate that the structural variations observed in these materials arise from the structure-directing effect of cations with different sizes. Additionally, this work reports their crystal structures, thermogravimetric analysis, infrared spectroscopy, UV-Vis-NIR diffuse reflectance spectroscopy, scanning electron microscopy/energy dispersive analysis by X-ray and electronic band structures. These new mixed-cation oxyhalide materials feature rich chemical and structural diversity as well as enhanced environmental stability, thus offering important material options for the development of infrared optical devices.