Birefringent crystals are key materials for generating and modulating polarized light and are widely used in optical communications, polarized information processing, high-precision scientific research instruments, and the laser industry. The design of current birefringent material that can be applied to deep ultraviolet (DUV) region (< 200 nm) remains a challenge. To meet the needs of scientific and technological development, the exploration of new DUV birefringent materials with excellent comprehensive performance is still urgently needed to be solved. It is found that planar unit [BO₃]^3？ with π-conjugate orbitals is the preferred fundamental building unit for birefringent materials in the short-wavelength range because they can provide high polarizability anisotropy and generate large birefringence.

In the preliminary work, the Research Center for Crystal Materials of Xinjiang Technical Institute of Physics and Chemistry used the classical dipole-dipole interaction model to study the B-O group, and systematically calculated the polarization anisotropy of the chained B_nO_2n+1 (n≥1) group polymerized by planar [BO₃]^3- elements. Team members concluded that the average polarizability anisotropy of the highly polymerized planar borate anionic groups, such as [B₂O₅]^4-, [B₄O₉]^6- and ¹_∞[BO₂] chain, are greater than that of the isolated [BO₃]^3-, the chain polymerization [BO₃]^3- primitive was proposed as an excellent structural motif for the design of large birefringent DUV crystals for the first time (J. Am. Chem. Soc., 2018, 140, 16311). A DUV birefringent crystal Li₂Na₂B₂O₅ containing coplanar arrangement [B₂O₅]^4- motifs, with a short UV cutoff edge (181 nm) and a large birefringence index (0.095@532nm), was designed by ion substitution strategy. Subsequently, Li₂Na₂B₂O₅ crystals with a size of 35 × 15 × 5 mm³ were grown by the top seed crystal method, and their physicochemical properties were systematically characterized (J. Am. Chem. Soc., 2019, 141, 3258). On this basis, we further studied the structural characteristics of the ¹_∞[BO₂] chain in depth and proposed that the better the planarity of the ¹∞[BO₂] chain, the more favorable the superposition of the effect of optical anisotropy. At the same time, LiBO₂ crystals with large birefringence (0.168@266 nm) and advancing the transmission cutoff to the DUV region down to 164 nm were selected and grown with high-quality single crystals up to Φ55 × 34 mm³ by combining the advantages of the alkali metal borate Li-B-O system (Light: Sci. Appl., 2022, 11, 252). The above studies provide new ideas for the design of DUV birefringent crystals.

Recently, researchers have continued their exploration in material systems containing ¹_∞[BO₂] chains and induced the formation of ¹_∞[BO₂] chains by introducing the more covalent AlO_mF_n (m + n = 4, 5, 6) motifs into the borates, providing a new way to design DUV birefringent crystals containing ¹_∞[BO₂] chain. A new fluoroaluminoborate Li_0.5Na_0.5AlB₂O₄F₂ was successfully synthesized using ion substitution and fluorination strategies, which contains a nearly coplanar arrangement of ¹_∞[BO₂] chains, and the AlO₃F₃ group was found for the first time in borates. Li_0.5Na_0.5AlB₂O₄F₂ with a short UV cutoff edge (< 200 nm) and a large birefringence (≥ 0.108 @ 546 nm), is a potential DUV birefringent material.

The related research result was published in full text in Advanced Optical Materials (Adv. Optical Mater. 2022, 2202353), with Xinjiang Technical Institute of Physics and Chemistry as the sole author, Professors Min Zhang and Shili Pan as corresponding authors, and Dr. Ziting Yan and M.S. Dongdong Chu as co-first authors. This research work was supported by the National Key R&D Program, the National Science Foundation of China, and the Youth Innovation Promotion Association of CAS.

Article link: https://onlinelibrary.wiley.com/doi/10.1002/adom.202202353

Schematic of crystal structure design strategy