Anion ratio-directed design of CaB4O5F4 and CaB6O8F4: [BO3]/[BO2F2] hybridization tailoring deep-ultraviolet nonlinear optical performance

Abstract

Deep ultraviolet (DUV) fluorooxoborates with a 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 feature anionic frameworks combining π-conjugated functional units [BO₃] with non-π-conjugated units [BO₃F], systems incorporating [BO₃] and [BO₂F₂] remain notably less explored. By performing crystal structure prediction and first-principles calculations, we predicted two new DUV NLO fluorooxoborates, namely, 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 for DUV NLO materials, exhibiting the shortest phase-matching wavelength of 157 nm. For CaB₆O₈F₄, when the [BO₃]/[BO₂F₂] ratio is 2, the shortest phase-matching wavelength is 163 nm. We note here that CaB₆O₈F₄ not only maintains a large bandgap (7.66 eV) but also exhibits a 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 study offers a promising hybridization tailoring strategy for the exploration of novel fluorooxoborate crystals through rational anion engineering.