NaBaBS₃: A Promising Infrared Functional Material with Large Birefringence Induced by π-Conjugated [BS₃] Units

The first thioborate in the Na–Ba–B–S system NaBaBS3 has been synthesized successfully, together with a new phase of LiBaBS3 (β-phase). NaBaBS3 shows great potential to be an infrared (IR) birefringence (Δn) material due to its large Δn = 0.243, wide band gap (3.96 eV), and high laser-induced damage threshold (5 × AGS). The results of structure–property relationship based on the response electron distribution anisotropy and real space atom cutting methods illustrate that the large birefringence is induced from the π-conjugated [BS3] units (about 90% contribution). Moreover, a series of thioborates (with [BS3] as the only anion group) were investigated about the birefringence, and the results indicate that [BS3] could be regarded as a good gene for designing IR birefringent materials.