Prediction of ultraviolet optical materials in the K₂O-B₂O₃ system

Ultraviolet (UV) birefringent crystals play a crucial role in various fields, such as laser technologies, optical telecommunications, and advanced scientific instrumentation. Alkali metal borates, with their diverse structures and remarkable ultraviolet optical properties, have garnered significant attention in recent years. In this study, employing the evolutionary crystal structure prediction algorithm USPEX, in conjunction with ionic substitutions and first-principles calculations, we systematically explored the pseudo-binary K₂O-B₂O₃ system and predicted two stable structures (oP56-K₃BO₃ and mC44-K₄B₂O₅) previously unreported, and twelve metastable structures in the K₂O-B₂O₃ system. A comprehensive analysis of their structural, electronic and optical properties is conducted. The coplanar arrangement of BO₃ and B₃O₆ groups is found to enhance optical anisotropy, thereby increasing the birefringence. In the K₂O-B₂O₃ system, six structures with wide band gaps and high birefringence (mP28-1-K₃BO₃, tR72-KBO₂, oP112-1-KB₅O₈, oP112-2-KB₅O₈, mC220-K₅B₁₉O₃₁, and hR21-K₃BO₃) are found to be possible candidates for UV optical materials. Importantly, hR21-K₃BO₃, the only non-centrosymmetric structure in this system, exhibits a significant frequency doubling coefficient (about 4.6 KDP) and a moderate birefringence index (0.056@1064 nm), marking it a promising UV nonlinear optical material.