Recently, the Crystal Materials Research Center at the XTIPC achieved a breakthrough in the study of solar-blind ultraviolet birefringent crystals. The research team innovatively proposed three molecular assembly strategies and, by precisely controlling the arrangement of functional units, successfully designed and synthesized nine new birefringent crystals. Among them, RbHC₂O₄, Rb₂C₂O₄·B(OH)₃, and Rb₂HCO₃F·B(OH)₃ exhibited excellent comprehensive properties, providing a new material system for the development of solar-blind ultraviolet polarization optical devices.

Birefringent crystals are key materials in fields such as laser modulation, polarization imaging, and optical communication. The solar-blind ultraviolet region (wavelength < 280 nm) offers unique advantages in military, communication, and detection applications due to its extremely low background interference. However, traditional commercial birefringent crystals such as MgF₂, CaCO₃, and α-BaB₂O₄ face issues like low birefringence or insufficient transmittance in the solar-blind ultraviolet region, limiting their further application. The Crystal Materials Research Center has long been dedicated to the performance regulation and structural design of optical crystal materials. In this study, researchers proposed three innovative molecular assembly strategies: (1) protonation modification of traditional π-conjugated groups, (2) reorganization of functional units through structural design, and (3) introduction of fluorine atoms to regulate structure and properties. The team systematically synthesized nine crystals and conducted in-depth research on their structure-property relationships.

The study found that three representative crystals exhibited outstanding optical properties: RbHC2O4 demonstrated a measured birefringence of 0.302 at 546 nm, one of the highest values among known alkali metal oxalates, along with a broad bandgap of 4.48 eV, easy crystal growth, and good stability; Rb₂C₂O₄·B(OH)₃ showed a birefringence of 0.195 and a bandgap of 4.68 eV, achieving a synergistic improvement in both bandgap and birefringence; Rb₂HCO₃F·B(OH)₃ exhibited a short ultraviolet cutoff edge of 185 nm and a birefringence of 0.116, making it a highly promising deep-ultraviolet birefringent crystal.

Theoretical studies further revealed that protonation modification effectively enhances the electron density distribution and polarization anisotropy of π-conjugated groups. Additionally, solvent regulation and hydrogen bond-directed assembly methods enable the ideal arrangement of functional units, significantly improving macroscopic birefringence performance. This study is the first to systematically experimentally demonstrate the regulatory mechanism of molecular assembly on the properties of solar-blind ultraviolet birefringent crystals, providing new ideas and methods for the directional design of high-performance polarization optical materials in the future.

The related research findings were published in full in the journal Small (Small, 21, 2504184). The Xinjiang Technical Institute of Physics and Chemistry was the sole corresponding institution, with Researcher Shilie Pan and Researcher Jian Han from the Crystal Materials Research Center serving as corresponding authors. Doctoral students Guangsheng Xu and Chenhui Hu were the first authors of the paper. The research was supported by the Tianshan Talent Training Program, Key Research and Development Program of Xinjiang, Strategic Priority Research Program of the Chinese Academy of Sciences, National Natural Science Foundation of China, and the Xinjiang Major Science and Technology Project.

Figure 1: Synthesis of optical crystals guided by birefringence under the molecular assembly strategy