Prediction of ultraviolet optical materials in the K2O-B2O3 system.

Abstract

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.