C2H12N6C4O4·2H2O and Na2C4O4·3H2O: Two Ultraviolet Birefringent Compounds with the [C4O4]2– Group

In modern optics, birefringent materials that can manipulate light polarization play important roles in lasers and information fields. The search for ultraviolet (UV) crystals with large birefringence is the focus of attention in the field of optical materials. In this work, we synthesized two birefringent crystals, C2H12N6C4O4·2H2O and Na2C4O4·3H2O, containing planar π-conjugated [C4O4]2– groups. Attributed to the large structural anisotropy and relatively ordered arrangement of the [C4O4]2– groups, C2H12N6C4O4·2H2O and Na2C4O4·3H2O possess large birefringence of 0.20–0.21 at 1064 nm. Meanwhile, they exhibit short ultraviolet cutoff edges at about 280–300 nm, corresponding to the large band gaps of 4.35 and 4.24 eV, respectively. Using structural analysis and first-principles calculations, the origins of such large birefringence are investigated and discussed. This work provides two potential UV birefringent crystals and prompts the search for novel birefringent materials.