RbCl·(H₂SeO₃)₂: A Salt-Inclusion Selenite Featuring Short UV Cut-Off Edge and Large Birefringence

Birefringent materials are key components to control the light polarization in laser science and technology as well as optical communication. However, the performance of current commercial birefringent materials has been limited by the magnitude of birefringence, optical transparency range, or the attainability of large-scale single crystals. To explore new birefringent materials, we strategically incorporated a lone pair cation (Se4+) with large optical anisotropy, an alkali metal, and halogen ions (Rb+ and Cl–) with superior UV transparent capacity; thus a new compound, RbCl·(H2SeO3)2, was successfully discovered with the aid of the facile hydrothermal method. Interestingly, Rb-Cl chains locate in the [H2SeO3]∞ skeleton, which makes RbCl·(H2SeO3)2 a salt-inclusion selenite. Millimeter-sized single crystals (up to 4 × 2 × 1 mm3) were obtained, and the transmittance spectrum revealed that its UV cut-off edge can be as low as 230 nm. In addition, the calculated birefringence of RbCl·(H2SeO3)2 is 0.14 at 589 nm that is similar to the birefringent value of famous α-BaB2O4. Wide UV transparency, large birefringence, and feasible crystal growth make RbCl·(H2SeO3)2 a new member of birefringent materials for UV light applications