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, Na 6 (SO 4 ) 2 SiF 6 . The introduction of the perfluorinated [SiF 6 ] group with a large HOMO–LUMO gap and the weakening of the d – p π-bond in the [SO 4 ] group by alkali metal cations widen the DUV transparency window. Meanwhile, the oriented arrangement of [SO 4 ] groups gives rise to the optimal superposition of their microscopic second-order susceptibilities to strengthen the SHG response. Remarkably, Na 6 (SO 4 ) 2 SiF 6 features the strongest SHG response (1.45 × KH 2 PO 4 ) among DUV transparent fluosilicate NLO crystals and a short DUV cutoff edge below 190 nm. Moreover, Na 6 (SO 4 ) 2 SiF 6 exhibits a wide transparency window of 0.19–8.45 μm, a large laser-induced damage threshold of 315 MW/cm 2, high thermal stability up to 400 °C under an air atmosphere, and feasible growth of centimeter-sized single crystals. This research indicates that Na 6 (SO 4 ) 2 SiF 6 has great application potential and provides a method to pave the way for the exploration of next-generation DUV NLO crystals.