Wide bandgaps and strong SHG responses of hetero-oxyfluorides by dual-fluorination-directed bandgap engineering

A wide bandgap is an essential requirement for a nonlinear optical (NLO) material. However, it is very challenging to simultaneously engineer a wide bandgap and a strong second-harmonic generation (SHG) response, particularly in NLO materials containing second-order Jahn-Teller (SOJT) distorted units. Herein, we employ a bandgap engineering strategy that involves the dual fluorination of two different types of SOJT distorted units to realize remarkably wide bandgaps in the first examples of 5d⁰-transition metal (TM) fluoroiodates. Crystalline A₂WO₂F₃(IO₂F₂) (A = Rb (RWOFI) and Cs (CWOFI)) exhibit the largest bandgaps yet observed in d⁰-TM iodates (4.42 (RWOFI) and 4.29 eV (CWOFI)), strong phase-matching SHG responses of 3.8 (RWOFI) and 3.5 (CWOFI) × KH₂PO₄, and wide optical transparency windows. Computational studies have shown that the excellent optical responses result from synergism involving the two fluorinated SOJT distorted units ([WO₃F₃]³⁻ and [IO₂F₂]⁻). This work provides not only an efficient strategy for bandgap modulation of NLO materials, but also affords insight into the relationship between the electronic structure of the various fluorinated SOJT distorted units and the optical properties of crystalline materials.