KNaSiF₆: 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 KNaSiF₆ (space group Pna2₁), 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 [SiF₆]^2- anionic groups, and energy-dispersive X-ray spectroscopy verifies the expected stoichiometry. First-principles calculations on the Pna2₁ 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 Pna2₁-KNaSiF₆ a promising candidate for next-generation DUV optical windows and nonlinear optical crystals.