Abstract
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.