From centrosymmetric to noncentrosymmetric: effect of the cation on the crystal structures and birefringence values of (NH4)n−2AE(PO2F2)n (AE = Mg, Sr and Ba; n = 2, 3 and 4)

Abstract

Fluorine-containing compounds significantly enrich the diversity of inorganic chemical structures. Here, four alkaline-earth metal difluorophosphates with different space groups, namely NH₄Mg(PO₂F₂)₃ (Cmcm) (1), NH₄Sr(PO₂F₂)₃ (P) (2), (NH₄)₂Ba(PO₂F₂)₄ (P2/n) (3) and Ba(PO₂F₂)₂ (I2d) (4), were synthesized and their structures were determined for the first time. 4 is not only the first alkaline-earth-metal difluorophosphate, but also the first difluorophosphate reported to crystallize in the non-centrosymmetric space group I2d of the tetragonal crystal system. All of the three-dimensional (3D) structures of the title compounds consist of isolated (PO₂F₂)⁻, AE–O and NH₄⁺ units, while the differences of the cation radius result in various arrangements of the (PO₂F₂)⁻ units, which leads to diverse crystal structures and the disparate optical anisotropy of the crystals. According to the deep-ultraviolet (DUV) transmittance spectrum, 3 exhibits a short DUV cut off edge (<180 nm). Solid state ¹⁹F and ³¹P magic-angle spinning NMR spectroscopy was used to verify the presence of covalent P–F bonds in 3. The different size effects, as well as the coordination environments of the Mg²⁺, Sr²⁺, and Ba²⁺ cations, on the structures as a whole have been discussed in detail. In addition, the calculated birefringence values were observed to differ from 0.011 to 0.033 for the four compounds.