Rational Design of the First Lead/Tin Fluorooxoborates MB₂O₃F₂ (M = Pb, Sn), Containing Flexible Two-Dimensional [B₆O₁₂F₆]_∞ Single Layers with Widely Divergent Second Harmonic Generation Effects

Abstract

Molecular engineering design is a productive atomic-scale strategy to optimize crystal structure and develop new functional materials. Herein, the first lead/tin fluorooxoborates, MB₂O₃F₂ (M = Pb, Sn), were rationally designed by employing the nonlinear optical crystal Sr₂Be₂B₂O₇ (SBBO) as a parent model. Compared with the rigid [Be₆B₆O₁₅]_∞ double layers in SBBO, MB₂O₃F₂ have flexible two-dimensional [B₆O₁₂F₆]_∞ single layer, which not only keeps the NLO-favorable layered structure but also overcomes the structural instability issues of SBBO. Both compounds exhibited desired short UV cutoff edge. Interestingly, MB₂O₃F₂ exhibit widely divergent second harmonic responses, although they are isostructural and both contain stereochemically active lone-pair cations. Our first-principles calculations revealed that the SHG difference is mainly attributed to the different anisotropies of Pb and Sn SHG-active orbitals, which make constructive and destructive contributions to the SHG effects in PbB₂O₃F₂ and SnB₂O₃F₂, respectively.