Sulfates have become an important research direction in the field of optical crystals due to their wide optical bandgap and short UV cutoff edges. The near orthotetrahedral structure of SO4 primitives has a high symmetry, which makes their polarizabilities less anisotropic and thus contributes little to the birefringence of the crystals, which hampers them from designing and applying in optical crystal materials. Constrained by the equilibrium relationship between bandgap and birefringence, how to enhance the birefringence while ensuring a wide bandgap is an urgent problem to be solved.

The team of Shilie Pan at the Xinjiang Institute of Physics and Chemistry Technology, Chinese Academy of Sciences, proposed a birefringence engineering strategy to optimize the optical properties of sulfate units through ligand substitution to achieve wide bandgap and large birefringence performance. The strategy starts by breaking the high symmetry of SO4 units and introducing different groups for ligand substitution. The exploration of crystal engineering ligand substitution based on pristine SO4 primitives involves 13 potential modular units. Compared to the pristine SO4 units, the new modules have improved anisotropy and satisfy the crystal engineering design goals. To verify the superiority of the design, the team assembled 15 compounds to evaluate the optical properties, of which nine crystal structures were discovered for the first time. In the experiments, centimeter-scale crystals of aminoguanidine sulfamate were grown and evaluated for optical properties such as birefringence. By ligand substitution, the birefringence of these derivatives is 4-6 times higher than that of the corresponding sulfates. The birefringence of aminoguanidinium sulfonate is about 35 times that of lithium sulfate, which significantly enhances the birefringence and confirms the superiority of the new paradigm. Unlike the conventional trial-and-error method, the new paradigm proposed in this study significantly enhances the birefringence of sulfate-derived materials by strategically tailoring SO4 units and implementing ligand substitutions to modulate the optical properties. By pre-evaluating the effect of ligand substitution in microscopic units, the proposed new paradigm improves the success rate of designing high-performance optical crystals, facilitates the development of novel birefringent crystals, and provides new ideas for the exploration of novel optical materials in the short-wavelength region.

The research results were published in ACS Central Science as a full research article, with Xinjiang Institute of Physics and Chemistry as the sole completion unit, Shilie Pan and Jian Han as the corresponding authors, and Chenhui Hu as the first author. This work was supported by the Ministry of Science and Technology, the National Natural Science Foundation of China, the Chinese Academy of Sciences, and the Science&Technology Department of Xinjiang Uyghur Autonomous Region.

The comparison for birefringence properties of the designed novel compound and the original compound.