The design of a non-centrosymmetric structure in sulfates by cation-induced symmetry breaking†

Since the non-centrosymmetric structure is a prerequisite for second-order nonlinear optical crystals, much attention should be paid to its design. Herein, a new strategy for cation modulation was proposed and implemented with the LiCsSO₄ structure as a matrix and the ammonium cation as a polar structure inducer. Then, a novel alkali metal sulfate, Li₂NH₄Cs(SO₄)₂, was obtained, which exhibits a polar structure with a space group of Pna2₁. The introduction of ammonium cations induces symmetry breaking with the disappearance of the inversion center. Moreover, based on the synergy of multiple cations, the large distortion of sulfate groups and their uniform arrangement occur, which directly brings about a large second harmonic generation response (1.70 × KH₂PO₄ under a 1064 nm laser). Therefore, this work demonstrates a new design strategy with cation modulation, which will facilitate the development of nonlinear optical crystals.