Аннотация:Single crystals of a new compound (NH4)2[Mg3(P2O7)2(H2O)2] belonging to the family of aqueous diphosphates with the general formula A2M3[P2O7]2(H2O)2 where A = K, NH4, Rb and Na, and M = Mn, Fe, Co, Ni were obtained for the first time by hydrothermal synthesis at T = 553 K and P = 7 MPa. The new compound has unit cell parameters a = 9.3047(2), b = 8.0843(2), c = 9.3655(3) Å, β =100.225(1)°. The model of the crystal structure was determined in the sp. gr. P21/c with accuracy to R = 0.0235. Raman spectra were obtained for the first time for compounds of this structural type, the presence of a water molecule in the (NH4)2[Mg3(P2O7)2(H2O)2] structure and the formation of complex P2O7 phosphate groups were confirmed.