α-BBO-like π-Conjugated Crystal with Large Birefringence

Birefringent crystals are in great demand for high performance polarizing devices. Designing new birefringent crystals with large birefringence remains a challenge. Herein, we successfully synthesized a new α-BBO-like π-conjugated birefringent crystal, K2HC3N3S3·1.5H2O (I). I exhibits a relatively large birefringence of 0.31@550 nm; such a birefringence is larger than those of most present commercial birefringent crystals. This result indicates that I might be a potential candidate for polarization modulation components in optical devices. First-principles calculations reveal that the planar π-conjugated [HC3N3S3]2– anions make dominant contributions in the large birefringence of I.