Modulation of Crystal Structure and Birefringence: Cation Size Effect in Alkali Metal Fluoroaluminoborates

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.