Large optical anisotropy-oriented construction of a carbonate-nitrate chloride compound as a potential ultraviolet birefringent material

The design of new birefringent materials is very significant owing to their indispensable role in modulating the polarization of light and is vital in laser technology. Herein, by applying a large optical anisotropy-oriented construction induced by a synergy effect of multiple anionic groups, a promising carbonate-nitrate chloride, Na₃Rb₆(CO₃)₃(NO₃)₂Cl·(H₂O)₆, has been designed and synthesized successfully by the solvent evaporation method and single crystals of centimeter size were obtained by the recrystallization method in aqueous solution. It crystallizes in the hexagonal P6₃/mcm space group, the RbO₉Cl polyhedra and the NaO₇ polyhedra construct a three-dimensional (3D) framework by sharing O or Cl atoms and trigonal plane units (CO₃ and NO₃). The transmittance spectrum based on a 1 mm thick single-crystal plate shows that its short UV cut-off edge is about 231 nm. And the refractive index differences (0.14 @ 546 nm) measured by using a polarizing microscope on the (101) crystal plane, proves that Na₃Rb₆(CO₃)₃(NO₃)₂Cl·(H₂O)₆ has a large birefringence, which has potential application in the solar blind ultraviolet region. The theoretical calculations reveal that the π-conjugated CO₃ and NO₃ groups are the main cause of the birefringence. It demonstrates that combining π-conjugated CO₃ and NO₃ groups in one structure is an extremely effective strategy to explore new UV birefringent crystals.