An Ultraviolet Transparent Nonlinear Optical Crystal Assembled from a Polarizable Tetrahedral Unit

Polarizable tetrahedra consisting of two distinct types of chemical bonds are of great interest in the design of nonlinear optical (NLO) crystals that are transparent in the ultraviolet (UV) and deep-UV regimes. Herein we report a hydrated barium methylsulfonate, Ba₂(CH₃SO₃)₄·3H₂O, that crystallizes in the noncentrosymmetric space group <i>Pna</i>2₁. Its crystal structure adopts a quasi-two-dimensional framework built from [BaO₇]/[BaO₈] polyhedra and polarizable [CH₃SO₃]^- tetrahedra. Centimeter-sized single crystals were obtained by using a facile solution-evaporation method. Optical measurements show that the title compound is UV-transparent with a short cutoff edge at 207 nm. It exhibits phase-matchable second-harmonic generation (SHG) in the visible region, with a moderate efficiency of 0.6 times that of KDP, and shows a birefringence value of 0.012 at 550 nm. First-principles calculations reveal that its optical properties originate mainly from the [CH₃SO₃]^- groups as well as H₂O molecules.