ASb(SO4)2 (A = Rb, Cs): Two short-wave UV antimony sulfates exhibiting large birefringence

Herein, two antimony sulfates, named RbSb(SO4)2 (1) and CsSb(SO4)2 (2), have been successfully synthesized with the introduction of Sb3+ cation with stereochemically active lone pairs (SCALP) into sulfates by the conventional hydrothermal method. Both two compounds endow short ultraviolet (UV) absorption edges (281 nm and 278 nm, respectively) and large birefringence (0.171@546 nm and 0.174@546 nm, respectively), which means that they are promising short-wave UV optical materials. Interestingly, though both of the two compounds exhibit similar 1D chained structures, and possess the same functional moieties including SbO4 seesaws and SO4 tetrahedral groups, they exhibit significantly opposite macroscopic symmetries, i.e., compound 1 crystallizes in a centrosymmetric (CS) manner (P21/n) and compound 2 in a noncentrosymmetric (NCS) manner (P212121), due to the size of cations [r(Rb+) = 1.56 Å, r(Cs+) = 1.67 Å] affects the orientation of SCALP of the adjacent Sb3+.