Giant Second-Harmonic Generation Response and Large Band Gap in the Partially Fluorinated Mid-Infrared Oxide RbTeMo₂O₈F

Strong second-harmonic generation (SHG) and a wide band gap are two crucial but often conflicting parameters that must be optimized for practical nonlinear optical (NLO) materials. We report herein the first d⁰-transition-metal (TM) tellurite with half of the d⁰-TM-octahedra partially fluorinated, namely, quinary RbTeMo₂O₈F, which exhibits giant SHG responses (27 times that of KH₂PO₄ (KDP) and 2.2 times that of KTiOPO₄ (KTP) with 1064 and 2100 nm laser radiation, respectively), the largest SHG values among all reported metal tellurites. RbTeMo₂O₈F also possesses a large band gap (3.63 eV), a wide optical transparency window (0.34-5.40 μm), and a significant birefringence (Δn = 0.263 at 546 nm). Theoretical calculations and crystal structure analysis demonstrate that the outstanding SHG responses can be definitively attributed to the uniform alignment of the polarized [MoO₅F]/[MoO₆] octahedra and the seesaw-like [TeO₄], and the consequent favorable summative polarization of the three distinct SHG-active polyhedra, both induced by partial fluorine substitution on the [MoO₆] octahedra.