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It is of great difficulty to create a new antimonite with second-harmonic-generation (SHG) intensity larger than 6 times that of KDP. In this study, a polyfluoroantimonite strategy has been proposed to explore fluoroantimonites with large nonlinear optical (NLO) coefficients. Under the cooperation of chemical (highly asymmetric π-conjugated organic amine) and physical (viscous reaction medium ethylene glycol) methods, two novel polyfluoroantimonites, namely, (3PC) 2 (Sb 4 F 14 ) and (3AP) 2 (Sb 4 F 13 ), have been achieved. Interestingly, these two structures contain two new polyfluoroantimonite groups respectively, an isolated (Sb 4 F 14 ) 2- four-member polyhedral ring and an infinite [Sb 4 F 13 ] ∞ - helical chain. More importantly, the polar (3AP) 2 (Sb 4 F 13 ) displays a strong SHG intensity of 8.1 × KDP, a large birefringence of 0.258@546 nm and a high laser-induced damage threshold (LIDT) value of 149.7 MW cm -2 . Theoretical calculations indicated that its strong SHG effect stems from the synergistic effect of the helical [Sb 4 F 13 ] ∞ - polyfluoroantimonite chain and π-conjugated 3AP + cation, with a contribution ratio of 48.93% and 50.77% respectively. This work provides a new approach for the design and synthesis of high-performance fluoroantimonites., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

An unusual O/F ordered d 0 transition metal fluoroantimonite, namely, K 2 SbMoO 2 F 7 , has been created by the cationic size effect of alkali metals. It features the largest birefringence of 0.220@550 nm among inorganic antimonites with a halogen element, which is an order of magnitude larger than the disordered A 2 SbMoO 2 F 7 (A = Rb, Cs). These three new compounds exhibit two different structures, although all of the structures were made of [SbMoO 2 F 7 ] 2- chains formed by SbF 5 square pyramids and MoO 2 F 4 octahedrons. A transparent single crystal of K 2 SbMoO 2 F 7 with dimensions of 7.0 × 5.0 × 1.0 mm 3 has been successfully grown by the aqueous solution volatilization method. The UV-vis-MIR transmission spectrum showed that K 2 SbMoO 2 F 7 can display excellent transmittance in the range of 0.5-5.0 μm and 6.0-9.8 μm, indicating its application potential as a birefringent material in the mid infrared band. This work offers a fresh approach to the design and synthesis of mid infrared birefringent materials.