KNaSiF 6 : a noncentrosymmetric deep ultraviolet optical crystal

Deep-ultraviolet (DUV) optical materials with wide bandgaps and non-centrosymmetric structures are critical for advanced optoelectronic applications such as DUV lithography, high-resolution spectroscopy and nonlinear optics. In this work, we report the discovery of a noncentrosymmetric phase of KNaSiF 6 (space group Pna 2 1 ), synthesized via a hydrothermal cation-exchange method. Single crystals with dimensions up to 4.45 mm × 1 mm × 1 mm were grown. The crystal exhibits an absorption edge below 200 nm, corresponding to a bandgap greater than 6.2 eV. Vibrational spectroscopy (IR and Raman) confirms the integrity of the [SiF 6 ] 2− anionic groups, and energy-dispersive X-ray spectroscopy verifies the expected stoichiometry. First-principles calculations on the Pna 2 1 phase yield a small second-harmonic generation (SHG) coefficient of ∼0.00227 pm V −1 , consistent with the weak SHG response observed experimentally. The combination of a DUV transparency window, a wide bandgap and a noncentrosymmetric structure renders Pna 2 1 -KNaSiF 6 a promising candidate for next-generation DUV optical windows and nonlinear optical crystals.