CsY(SO 4 ) 2 ·4H 2 O: A Deep-Ultraviolet Birefringent Crystal Induced by an Edge-Sharing Connection.

SO 4 tetrahedral groups have weak polarization anisotropy, which thus results in the small birefringence of sulfates. Here, we report new sulfate CsY(SO 4 ) 2 ·4H 2 O with unprecedented birefringence among deep-ultraviolet (deep-UV) sulfates. Its single crystal (10 mm × 3.5 mm × 1.5 mm) was simply grown by an aqueous solution evaporation technique, and it features a rare layered structure composed of YO 9 polyhedra, SO 4 tetrahedra, and H 2 O molecules. Interestingly, each SO 4 group donates two oxygen atoms to edge-share with one adjacent YO 9 polyhedron and thus causes severe distortion of these groups. The characteristic edge-sharing mode gives CsY(SO 4 ) 2 ·4H 2 O a large birefringence of ∼0.045@546 nm, which is the maximum among deep-UV sulfates and phosphates with similar non-π-conjugated anionic groups. The ultraviolet-visible-near-infrared diffuse reflection and transmission spectra, infrared spectrum, thermal stability, and theoretical calculations are also presented. The fascinating results will improve our understanding of sulfates and may provide useful insights into the exploration of deep-UV sulfates with large birefringence.