Oxygenation strategies provide an easy route to long wave infrared layered crystals

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

Metal halides containing lone pair electrons are promising candidates for long-wave infrared resistant materials. However, especially when coordinated with heavy halogens, the “holodide” coordination configuration of metal ions significantly reduces the activity of the lone electrons and reduces the birefringence of the structure.

In a study published in Angewandte Chemie International Edition, a team of researchers led by Professor Kong Fang of the Fujian Institute on Structure of the Chinese Academy of Sciences discovered an easy route to long wave infrared crystals.

The researchers proposed an oxygenation strategy. This strategy can replace the sole halide ions of halogen polyhedron with divalent oxygen ions, activating isolated feed electrons in the central cation, facilitating crystal layer boundaries.

Based on the RB+-SB3+-Cl- system, the researchers acquired three new structures: RB13SB8CL37, RB3SB2OCL7, and RB2SB2OCL6. As the CL/SB ratio decreased in structure, SB3+ coordination geometry transitioned from a “perforated” octahedron to a “hemididided” square pyramid.

With the introduction of oxygen ions, the Sb3+ ions of RB3SB2OCL7 and RB2SB2OCL6 adopted a square pyramidal geometry with steric activity. These two compounds represent the first example of alkali metal antimonide (III) oxyhalide.

Additionally, the researchers grew large crystals (6×6×2 mm3) of RB2SB2OCL6. Due to the low oxygen content, the infrared cutoff edge of this crystal reached 14,380 nm and RB2SB2OCL6, showing excellent transmission performance in the 0.4-13.5 μM range.

Furthermore, the researchers found that the birefringence of the three new compounds was negatively correlated with the Cl/SB ratio, with the birefringence of Rb2Sb2ocl6 reaching 0.191 @550 nm, reaching 11.2 times the birefringence of RB13SB8CL37 (0.017 @550 nm). Therefore, RB2SB2OCL6 is a promising long-wave infrared layered crystal.

This study developed a new material system, alkali metal antimony(III) oxyhalide. This study explores the relationship between geometric structures, electronic structures, and optical properties of isolated electron-containing metal oxyhalides, and provides a new strategy for the development of long-wave infrared layered crystals.