Dual-Anion Strategy Induces Dual Enhancement Toward Ultrashort Phase-Matching Wavelength in Deep-UV Transparent d⁰ Transition Metal Oxyfluorides

The d0 transition metal oxides are the most commonly used nonlinear optical (NLO) materials in the visible light region; however, their limited band gaps seriously hinder their application in ultraviolet (UV) and deep-ultraviolet (DUV) regions. Achieving the double enhancement of band gap and birefringence by regulating anionic units helps to push their phase-matching (PM) wavelength into UV/DUV regions. Herein, starting from the famous NLO material LiNbO3, a "dual-anion strategy" is proposed to regulate the [NbO6–xFx] octahedra, and the predicted Li2Nb2O6–xF2x·(LiF)y (x = 1, 2, 4; y = 0, 2) materials exhibit the dual-property magnification of wide band gaps (3.82–6.26 eV, 1–3 eV larger than LiNbO3) and extraordinary birefringence (0.100–0.322, 1–4 times that of LiNbO3), along with a strong second harmonic generation (SHG) response of 2.6–6.2 × KDP. Remarkably, Li2NbOF5-I and LiNbOF4-II have extremely short PM wavelength (λPM = 209 nm) ever reported for d0 transition metal oxyfluorides. Further analysis uncovers that the fluorinated modification of band edges and the increase of octahedral anisotropy in [NbO6−xFx] anionic groups are the main reasons for the enhanced PM ability.