Anion Ratio-Directed Design of CaB4O5F4 and CaB6O8F4: [BO3] / [BO2F2] Hybridization Tailoring Deep-Ultraviolet Nonlinear Optical Performance

Abstract

Deep ultraviolet (DUV) fluorooxoborates with large second harmonic generation (SHG) response and suitable birefringence are preferred materials for new DUV nonlinear optical (NLO) crystals which are urgently needed in modern laser technology. While most reported fluorooxoborates with DUV phase matching abilities, featured anionic frameworks combining π-conjugated functional unit [BO3] with non-π-conjugated unit [BO3F], systems incorporating [BO3] and [BO2F2] remain notably under explored. Through performing crystal structure prediction and first-principles calculations, we predicted two new DUV NLO fluorooxoborates CaB4O5F4 and CaB6O8F4 whose anionic architectures uniquely integrate [BO3] and [BO2F2] units. CaB4O5F4 demonstrates a 1:1 [BO3]/[BO2F2] stoichiometric ratio, a structural configuration previously unreported in DUV NLO materials, achieving the shortest phase-matching wavelength of 157 nm. While for CaB6O8F4, when the [BO3]/[BO2F2] ratio is 2, the shortest phase-matching wavelength is 163 nm. We here note that CaB6O8F4 not only maintains a large bandgap (7.66 eV) but also exhibits large birefringence (0.106 at 1064 nm) and SHG response (3.5×KDP). Further analysis indicates that a well-designed combination of [BO3] and [BO2F2] units can effectively modulate the bandgap, birefringence, and SHG response. This work offers a promising hybridization tailoring strategy for the exploration novel fluorooxoborate crystals through rational anion engineering.