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)

Four alkaline-earth metal difluorophosphates with different space groups were synthesized for the first time. The differences in arrangements of the (PO₂F₂) units lead to disparate optical anisotropy of the crystals.