Computational design of new DUV NLO fluorooxoborates with [BO₃] : [BO₃F] = 2 : 1 via an anionic framework...

Fluorooxoborates are promising candidates for deep-ultraviolet (DUV) nonlinear optical (NLO) materials, with the [BO₃] : [BO₃F] ratio critically regulating the band gap, birefringence, and second-harmonic generation (SHG) effects. However, DUV NLO fluorooxoborate with a [BO₃] : [BO₃F] ratio of 2 : 1 has not yet been reported. In this work, with the [BO₃] : [BO₃F] ratio of 2 : 1, we conducted a targeted structural search while fixing the dimensionality of the anionic framework as [B₃O₅F] chains and [B₆O₉F₂] layers and applied to the calcium fluorooxoborate system. We successfully predicted three new dynamically stable non-centrosymmetric phases of CaB₃O₅F and CaB₆O₉F₂ that exhibit DUV phase-matching (PM) ability. The pronounced electron density difference along the optical principal axis within the flattened [B₆O₉F₂] layer in CaB₆O₉F₂-I results in the largest birefringence (0.101 at 1064 nm), making CaB₆O₉F₂-I exhibit a full-wavelength PM down to 162 nm. The aligned arrangement of functional units in the [B₃O₅F] chain leads to considerable SHG response of CaB₃O₅F-IV (1.124 pm V^-1), which is comparable to that of fluorooxoborates with higher [BO₃] : [BO₃F] ratios. This work fills the gap in the field of DUV NLO fluorooxoborates with a [BO₃] : [BO₃F] ratio of 2 : 1 and provides guidance for the design of DUV NLO materials.