Rational Design of the First Lead/Tin Fluorooxoborates MB 2 O 3 F 2 (M = Pb, Sn), Containing Flexible Two-Dimensional [B 6 O 12 F 6 ] ∞ Single Layers with Widely Divergent Second Harmonic Generation Effects.

Molecular engineering design is a productive atomic-scale strategy to optimize crystal structure and develop new functional materials. Herein, the first lead/tin fluorooxoborates, MB2 O3 F2 (M = Pb, Sn), were rationally designed by employing the nonlinear optical crystal Sr2 Be2 B2 O7 (SBBO) as a parent model. Compared with the rigid [Be6 B6 O15 ]∞ double layers in SBBO, MB2 O3 F2 have flexible two-dimensional [B6 O12 F6 ]∞ 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, MB2 O3 F2 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 PbB2 O3 F2 and SnB2 O3 F2 , respectively.