Na₆(SO₄)₂SiF₆: A Stable Sulfate Fluosilicate with Wide Deep-Ultraviolet Transparency Window and Strong Second-Harmonic Generation Response

During the development of deep-ultraviolet (DUV) nonlinear optical (NLO) crystals, how to better balance a wide DUV transparency window and strong second-harmonic generation (SHG) response has always been a challenge because they are usually contradictory in one NLO crystal. Herein, we report the design and synthesis of the first DUV transparent sulfate fluosilicate NLO crystal, Na6(SO4)2SiF6. The introduction of the perfluorinated [SiF6] group with a large HOMO–LUMO gap and the weakening of the d–p π-bond in the [SO4] group by alkali metal cations widen the DUV transparency window. Meanwhile, the oriented arrangement of [SO4] groups gives rise to the optimal superposition of their microscopic second-order susceptibilities to strengthen the SHG response. Remarkably, Na6(SO4)2SiF6 features the strongest SHG response (1.45 × KH2PO4) among DUV transparent fluosilicate NLO crystals and a short DUV cutoff edge below 190 nm. Moreover, Na6(SO4)2SiF6 exhibits a wide transparency window of 0.19–8.45 μm, a large laser-induced damage threshold of 315 MW/cm2, high thermal stability up to 400 °C under an air atmosphere, and feasible growth of centimeter-sized single crystals. This research indicates that Na6(SO4)2SiF6 has great application potential and provides a method to pave the way for the exploration of next-generation DUV NLO crystals.