Highly Birefringent d 0 Transition Metal Fluoroantimonite in the Mid Infrared Band: Order-Disorder Regulation by Cationic Size.

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.