Long-wave infrared birefringent crystals are essential materials in infrared optical applications in fields such as infrared imaging, laser technology, and optical communications. Due to limitations in birefringence, infrared transmission, and crystal growth, high-performance long-wave infrared birefringent crystals have rarely been reported. 

Metal halides containing lone-pair electrons are promising candidates for long-wave infrared birefringent materials, however, the "holodirected" coordination configuration of metal ions, particularly when coordinated with heavy halogens, significantly reduces the activity of the lone-pair electrons, thus lowering the birefringence of the structure.

In a study published in Angewandte Chemie International Edition, a research team led by Prof. KONG Fang from Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences discovered a facile route to long-wave infrared birefringent crystal, which is expected to overcome the trade-off between wide infrared transmission and large birefringence.

Researchers proposed an oxygenation strategy, wherein the monovalent halide ion in the halogen polyhedron can been replaced by a divalent oxygen ion, to activate the lone-pair electrons of the central cations, thus enhancing the birefringence of the crystal.

Based on Rb⁺-Sb³⁺-Cl^- system, researchers obtained three new structures, namely, Rb₁₃Sb₈Cl₃₇, Rb₃Sb₂OCl₇, and Rb₂Sb₂OCl₆. As the Cl/Sb ratio decreased in the structure, the Sb³⁺ coordination geometry transitioned from the "holodirected" octahedron to the "hemidirected " square pyramid. Due to the introduction of oxygen ions, the Sb³⁺ ions in Rb₃Sb₂OCl₇ and Rb₂Sb₂OCl₆ adopted a square pyramidal geometry with steric activity, and these two compounds represented the first examples of alkali metal antimony(III) oxyhalides.

Moreover, researchers grew a large crystal of Rb₂Sb₂OCl₆ (6×6×2 mm³). Due to the low oxygen content, the infrared cutoff edge of this crystal reached 14380 nm and Rb₂Sb₂OCl₆ can exhibited good transmission performance in the range of 0.4-13.5 μm, which was superior to many reported birefringent crystals. 

Furthermore, researchers found that the birefringence of the three new compounds was negatively correlated with the Cl/Sb ratio, and the birefringence of Rb₂Sb₂OCl₆ reached 0.191 @550 nm, which is 11.2 times that of Rb₁₃Sb₈Cl₃₇ (0.017 @550 nm). Therefore, Rb₂Sb₂OCl₆ is a promising long-wave infrared birefringent crystal.

This study not only develop a new material system—alkali metal antimony(III) oxyhalides. This study explores the relationship between the geometric structure, electronic structure, and optical properties of lone-pair electron-containing metal oxyhalides, providing a new strategy for the development of long-wave infrared birefringent crystals.

The large crystal and transmission spectrum of Rb₂Sb₂OCl₆, the birefringence and coordination mode of Sb³⁺ of Rb₁₃Sb₈Cl₃₇, Rb₃Sb₂OCl₇, and Rb₂Sb₂OCl₆. (Image by Dr. LI Xinyang)