Metal Methanesulfonates with Mixed Anionic Groups with Large Band Gaps and Enhanced Birefringence

Four new metal methanesulfonates with mixed anionic groups, Bi(SeO3)(CH3SO3) (1), Bi(TeO3)(CH3SO3) (2), Ba(BF4)(CH3SO3) (3), and Ba2(BF4)(CH3SO3)3(H2O)3 (4), have been successfully synthesized. Compounds 1 and 2 are isomorphic and exhibit a two-dimensional (2D) layered structure. Stereoactive lone pairs are observed with Se4+, Te4+, and Bi3+. Compound 3 features a three-dimensional (3D) structure formed by connecting [Ba(CH3SO3)]+ layers via BF4 tetrahedra, whereas compound 4 features a 2D layered structure, where the BF4 tetrahedra are positioned on both sides of the [Ba2(CH3SO3)3]+ layers. The ultraviolet–visible–near-infrared (UV–vis-NIR) spectra indicate that compounds 1 and 2 have absorption edges at 270 and 274 nm, whereas compounds 3 and 4, with absorption edges below 200 nm, are characterized as deep-UV crystals. Calculations were performed to determine the absorption edges, and comparative analysis indicates a significant enhancement in the calculated birefringence values compared to analogous compounds.