Broadband infrared-transparent crystals enabled by heterologous isomorphic substitution

Abstract

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.