AGa₃F₆(SeO₃)₂(A = Rb, Cs): A new type of phase-matchable hexagonal tungsten oxide material with strong second-harmonic generation responses

Combining a wide band gap, strong second-harmonic generation (SHG), and good phase matchability into a single structure is an ongoing challenge in the design of nonlinear optical (NLO) materials. Herein, we report the first examples of fluorinated gallium selenites AGa3F6(SeO3)2 [A = Rb (RGFS), Cs (CGFS)] by introducing fluorinated main-group metal octahedra [GaO2F4] into the metal selenite system. The two new compounds are isostructural (noncentrosymmetric space group P63mc) and feature interesting hexagonal tungsten oxide (HTO)-type [Ga3F6(SeO3)2]∞ layers, with alkali metal cations acting as linkers between these layers while also maintaining charge balance. Photophysical studies demonstrate that RGFS and CGFS exhibit strong phase-matchable SHG responses [5.6 × KH2PO4 (RGFS) and 5.4 × KH2PO4 (CGFS) at 1064 nm], high laser damage thresholds [39.37 MW/cm2 (RGFS), 41.59 MW/cm2 (CGFS) at 1064 nm, 10 ns pulses, 1 Hz repetition rate], wide band gaps [3.57 eV (RGFS), 3.65 eV (CGFS), the latter the highest value among polar HTO-type selenites or tellurites], and large birefringences (0.097 and 0.092 at 546 nm for RGFS and CGFS, respectively); the two compounds simultaneously exhibit multiple key NLO parameters for practical applications in the near-ultraviolet and mid-infrared region. The critical role of the [GaO2F4]5- groups in the linear and NLO responses in the two materials has been elucidated by first-principles studies. Our report demonstrates that introducing fluorinated main-group metal octahedra into metal selenite systems can lead to promising multicomponent selenite NLO materials.