Birefringent materials are a pivotal category of optical functional materials; however, finding a new birefringent material that can operate in the deep-ultraviolet (DUV) region remains elusive. Through structural integration of B3O6 with AlO3F units, fluoroaluminoborates have emerged as a chemically tunable platform for engineering birefringent materials. In this study, we report the first demonstration of cation-size-driven crystal structure evolution and birefringence modulation within fluoroaluminoborate systems. Two polymorphic potassium fluoroaluminoborates, α- and β-KAlB3O6F, were successfully synthesized through a high-temperature melting method. Both phases exhibit remarkable DUV optical performance, achieving wide band gaps coupled with exceptional birefringence (α: 0.093 @ 546 nm, β: 0.110 @ 546 nm). By combining the crystal structure analysis and the first-principles calculation results, we reveal cation-size-dependent alignment of B3O6/AlO3F groups as the origin of birefringence enhancement. This work establishes an effective strategy for simultaneously optimizing birefringence while maintaining DUV transparency through cation structural engineering.
Wei et al. (2025) studied this question.