A Rare-Earth-Based Borate Optical Crystal with Enlarged Dihedral Angles of Distinct [BO] Groups Exhibiting a Wide UV Transparent Range.

Borates, as advanced optical materials, have garnered wide interest due to their diverse structural configurations and great potential for applications in the ultraviolet (UV) regions. Herein, we synthesized a new rare-earth borate crystal, namely, KNaYBO, which is classified as one of the ABReBO compounds, where A and B represent alkali metal and Re denotes rare-earth metal. KNaYBO adopts in the monoclinic space group 2/ (No. 14), showcasing a three-dimensional (3D) framework composed of a planar triangular configuration of [BO] units and distortive [YO] polyhedra. Notably, both dihedral angles between distinct [BO] units reach 79.6°, which represents an unprecedented structural feature in monoclinic ABReBO-type crystals. Moreover, the compound has a short UV absorption edge at around 204 nm, corresponding to a wide band gap of approximately 5.67 eV. Additionally, it possesses a moderate birefringence of 0.028 at 1064 nm. Further analysis utilizing theoretical calculations suggests that the optical behaviors of KNaYBO are mainly governed by its basic structural unit [BO] triangles and distorted [YO] polyhedra. These findings enrich the structure chemistry of rare-earth borates and offer valuable insights for the design of optical crystals in the UV wavelength range.