Broadband infrared-transparent crystals enabled by heterologous isomorphic substitution

Broadband infrared-transparent crystals that simultaneously possess a short ultraviolet cut-off edge and extended infrared transmission are highly desirable for optical applications but remain challenging due to the inverse relationship between bandgap and infrared transparency. Here, we report a simple but effective heterologous isomorphic substitution strategy to address these issues. Using chalcogenide Ba₃GaS₄I as a structural template, we synthesized a series of inorganic halides A₃ZnCl₄I (A = K, Rb, NH₄) via room-temperature aqueous methods. Remarkably, these compounds are isostructural to the Ba₃GaS₄I parent compound, where [ZnCl₄] tetrahedra isomorphically replace [GaS₄] units, K ions substitute Ba ions, and I ions remain in their identical crystallographic position. This functional unit substitution significantly expands the transmission range: the ultraviolet cut-off edge blue-shifts from 322 nm in Ba₃GaS₄I to 236-247 nm in A₃ZnCl₄I (A = K, Rb, NH₄), while the infrared absorption edge extends from 13.2 μm to 17.5-17.7 μm. This work demonstrates that heterologous isomorphic substitution is an effective approach for modulating electronic structures and designing broadband infrared-transparent optical materials.