Explorations of highly birefringent materials in the vanadium oxyfluoride–iodate system by fluoride ion modulation

Abstract

Birefringent materials are of great interest because of their ability to manipulate light. The demand for smaller devices has driven the development of new birefringent materials with high levels of birefringences and overall excellent physicochemical properties. Two such materials, Sr[VO₂F(IO₃)₂] and Sr₃F₂(VO₂F₄)(IO₃), have been successfully synthesized by exploring the VOF polyhedron–iodate system. The compounds exhibit remarkable birefringence values of 0.250 and 0.406 at 550 nm, respectively, which are significantly higher than those of the commercially available birefringent vanadate (YVO₄, 0.204 at 532 nm), and the compound Sr₃F₂(VO₂F₄)(IO₃) has the largest birefringence in the iodate-fluoride system. Sr[VO₂F(IO₃)₂] is composed of polymerized anionic groups, [VO₂F(IO₃)₂]²⁻, while Sr₃F₂(VO₂F₄)(IO₃) is composed of optimally arranged IO₃⁻ and fluorinated VO₂F₄³⁻ functional groups organized by the Sr₃F₂⁴⁺ positively charged structure-oriented templates. The compounds have a wide optical transmission range of 0.28–10.7 μm and 0.43–10.3 μm, respectively. Moreover, they exhibit high thermal stability with values of 366 and 319 °C, respectively. These properties make them suitable for use in the middle-wavelength infrared region. The study demonstrates that hybridizing anionic functional groups and modulating the structure with anions are effective crystal engineering strategies for developing high-performance inorganic optical materials.