BaSbBS₄: a record-high-performance birefringent crystal identified by a target-driven closed-loop strategy

Exploring infrared (IR) birefringent materials with both large birefringence (Δ<i>n</i>) and wide band gaps (<i>E</i> _g) is urgently demanded for high-power optoelectronic applications and has long been a tough challenge due to the intrinsic contradictory relationship between the two metrics. Herein, we developed a target-driven closed-loop framework in coupling with functional motif and crystal structure screening, deep learning assisted high-throughput optical property computation, targeted experiment and mechanism investigation, enabling efficient discovery of potential birefringent materials. Utilizing it, a batch of superior IR birefringent crystals containing planar [BS₃]^3- and/or stereochemically active lone pair (SCALP) groups ([SbS₃]^3-, [SnS₃]^4-, <i>etc.</i>) were identified: six with huge birefringence (Δ<i>n</i> > 1.0) and three with both large birefringence (Δ<i>n</i> > 0.5) and wide band gaps (<i>E</i> _g > 3.5 eV). Remarkably, benefiting from a maximal synergy of [BS₃]^3- and [SbS₃]^3- motifs achieved by an optimal assembly of 1D [SbBS₄]_∞ chains, BaSbBS₄ was highlighted and then validated experimentally as the most promising IR birefringent crystal, unlocking a record high birefringence in the wide-band-gap range (Δ<i>n</i> = 0.95 & 2.70 eV). This work not only discovers new high-performance birefringent crystals but also offers a universal avenue for precise and efficient evaluation of optical functional materials.