Highly Birefringent d⁰ Transition Metal Fluoroantimonite in the Mid Infrared Band: Order–Disorder Regulation by Cationic Size

An unusual O/F ordered d⁰ transition metal fluoroantimonite, namely, K₂SbMoO₂F₇, 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 A₂SbMoO₂F₇ (A = Rb, Cs). These three new compounds exhibit two different structures, although all of the structures were made of [SbMoO₂F₇]²⁻ chains formed by SbF₅ square pyramids and MoO₂F₄ octahedrons. A transparent single crystal of K₂SbMoO₂F₇ with dimensions of 7.0 × 5.0 × 1.0 mm³ has been successfully grown by the aqueous solution volatilization method. The UV-vis-MIR transmission spectrum showed that K₂SbMoO₂F₇ 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.