K2M(H2P2O7)2.2H2O (M = Ni, Cu, Zn): orthorhombic forms and Raman spectra.

The crystal structures of three isotypic orthorhombic dihydrogendiphosphates, namely dipotassium copper(II)/nickel(II)/zinc(II) bis(dihydrogendiphosphate) dihydrate, K 2 M(H 2 P 2 O 7 ) 2 .2H 2 O (M = Cu, Ni and Zn), have been refined from single-crystal data. The M 2- and K + cations are located at sites of m symmetry, and the P atoms occupy general positions. These compounds also exist in triclinic forms with very similar structural features. The structures of both forms are compared, as well as the geometry of the MO 6 octahedron, which is considerably elongated towards the water molecules for M = Ni and Cu. Such elongation has not been observed among the other representatives of the family. A Raman study of the whole series K 2 M(H 2 P 2 O 7 ) 2 .2H 2 O (M = Mn, Co, Ni, Cu, Zn and Mg) is reported.