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 (P1[combining macron]) (2), (NH4)2Ba(PO2F2)4 (P2/n) (3) and Ba(PO2F2)2 (I4[combining macron]2d) (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 I4[combining macron]2d 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.