AB C Q X: A Thioborate Halide Family for Developing Wide Bandgap Infrared Nonlinear Materials by Coupling Planar [BS] and Polycations.

Developing high-performance infrared (IR) nonlinear optical (NLO) materials is urgent but challenging due to the competition between NLO coefficient and bandgap in one compound. Herein, by coupling NLO-active [BS] planar units and halide-centered polycations, six new metal thioborate halides ABaBSX (A = Rb, Cs; X = Cl, Br, I) composed of zero-dimensional [XBaRb/Cs] polycations and [BS] units, belonging to a new family, are rationally designed and fabricated. The compounds show an interesting structural transition from Pbcn (ABaBSCl) to Cmc2 (ABaBSBr and ABaBSI) driven by the clamping effect of polycationic frameworks. ABaBSBr and ABaBSI are the first series metal thioborate halide IR NLO materials, and the introduction of [BS] unit effectively widens the bandgap of planar unit-constructed chalcogenides. ABaBSBr and ABaBSI, exhibiting wide bandgaps (3.55-3.60 eV), high laser-induced damage thresholds (≈ 6 × AgGaS), and strong SHG effects (0.5-0.6 × AgGaS) with phase-matching behaviors, are the promising IR NLO candidates for high-power laser applications. The results enrich the chemical and structural diversity of boron chemistry and give some insights into the design of new IR NLO materials with planar units.