A₂MoO₂F₃(IO₂F₂) (A = Rb, Cs): Strong Nonlinear Optical Responses and Enlarged Band Gaps through Fluorine Incorporation

Tailored structural modulation to combine a broad optical band gap, large nonlinear optical (NLO) effect, and moderate birefringence in a single molecule is a crucial challenge for functional NLO material design. In this study, two first fluorinated molybdate fluoroiodates A2MoO2F3(IO2F2) [A = Rb (RMOFI), Cs (CMOFI)] were successfully accomplished by combining fluorinated d0-transition metal (TM) octahedra [MoO3F3] with fluoroiodate group [IO2F2]. These two compounds crystallize in noncentrosymmetric space group Cmc21 while featuring a zero-dimensional (0D) [MoO2F3(IO2F2)]2- functional motif, and their respective alkali-metal cations act as linkers in polyanions. Both RMOFI and CMOFI possess large second-harmonic generation (SHG) effects (5.0 × KH2PO4 (RMOFI) and 4.5 × KH2PO4 (CMOFI) at 1064 nm), wide band gaps (3.77 eV (RMOFI) and 3.43 eV (CMOFI)), sufficient birefringences (0.217 (RMOFI) and 0.204 (CMOFI) at 1064 nm) to achieve phase-matchable (PM) and broad infrared (IR) transparency region, suggesting that RMOFI and CMOFI can be promising NLO materials in the IR field. Theoretical calculations elucidate that the strong SHG responses and enlarged birefringences of the title compounds originate from the 0D [MoO2F3(IO2F2)]2- functional unit. This is in sharp contrast to the previous work that most fluoroiodates adopt centrosymmetric structures. Our study illustrates that introducing fluorinated d0-TM octahedra into fluoroiodate systems can be a facile method for developing high-performance nonlinear optical materials.