HgB₂S₄: A d¹⁰ Metal Thioborate with Giant Birefringence and Wide Band Gap

Birefringent crystals are important optical devices in various advanced optical systems because they can create and control polarized light. In this work, the capability of planar [B₃S₆] to produce large birefringence was theoretically studied for the first time. Moreover, the first d¹⁰ metal thioborate HgB₂S₄ (HBS) has been synthesized successfully, which was constructed by the π-conjugated [B₃S₆] and [HgS₂] units. Notably, HBS shows great potential to be an infrared birefringent material due to its large birefringence, Δn = 0.52 @ 1064 nm (∼4.3 × α-BaB₂O₄ and ∼10.4 × AgGaS₂ @ 1064 nm), wide band gap of 3.36 eV, and wide transparent window (∼0.37-25 μm). To the best of our knowledge, both the birefringence and the band gap of HBS are the largest among mercury sulfides. Structure-performance analyses reveal that the π-conjugated [B₃S₆] units are the main birefringence contributor and could be regarded as a good unit for designing IR birefringent materials. This work will provide direct guidance to design high performance birefringent crystals in the future.