Regulation of Refractive Index by Halogens in Two-Dimensional Layered Methylsulfonate Halides

Herein, a series of unprecedented alkaline earth metal methylsulfonate halides, M(SO₃CH₃)X·nH₂O (M = Sr, Ba; X = Cl, Br, I; n = 2, 4), and Ba₂(SO₃CH₃)₃I·4H₂O were synthesized. They all feature two-dimensional (2D) layered structures with halogen elements inserted between the 2D layers constructed by metal ions and [SO₃CH₃] groups, which are suitable for exploring the influence of halogens on their refractive index. The refractive indexes both parallel and (almost) perpendicular to the 2D layers were calculated through theoretical calculations. We found that the nonpolar halogens exhibited different enhancement in the two directions. The result may guide regulating the birefringence properties of such 2D crystal materials. Besides, the ultraviolet (UV) cutoff edge of Sr(SO₃CH₃)Cl·4H₂O and Ba(SO₃CH₃)Cl·4H₂O can reach 183 and 184 nm, respectively, indicating their potential application in the deep-UV region.