ACID DIPHOSPHATES M2(H2P2O7)2 (M = Ni, Co, Zn)-CRYSTAL STRUCTURE AND DEHYDRATION BEHAVIOUR
2002 Volume 13 Pages 83-86
Details
2002 Volume 13 Pages 83-86
Microcrystalline powders of acid diphosphates M2(H2P2O7)2 (M = Co, Ni, Zn) deposit from solutions of the metals in half-concentrated H3PO4 (nM : nP~1: 100) at T = 70°C. Yellowish single crystals of Ni2(H2P2O7)2 suitable for X-ray investigations have been obtained in reactions under hydrothermal conditions (VAmp.~2.5 mL, T~400°C, 100 mg NiO, 125 mg P4O10, 1 mL conc. H3PO4). The crystal structure of Ni2(H2P2O7)2 (P21/c, Z = 4, 20 non-hydrogen atoms in the asymmetric unit, 2 of 4 hydrogen atoms located from Δ-Fourier synthesis, R1 = 0.027 for 1696 Fo > 4σ(Fo), wR2 = 0.067, 2530 reflections, 190 parameters) consists of infinite chains of edge-sharing [NiO6] octahedra and [H2P2O7] units. Two diphosphate groups are linked by hydrogen bridges, thus building an arrangement similar to a cyclo-tetrametaphosphate. A second type of hydrogen bridges is suggested to connect these rings to a 3-dimensional network. Between the structures of the M2(H2P2O7)2 and anhydrous cyclo-tetrametaphosphates M2P4O12 a remarkably close relation is observed, suggesting a topotactic mechanism for the dehydration reaction. Actually, upon heating of Zn2(H2P2O7)2 to 400°C Zn2P4O12, which is metastable with respect to Zn(PO3)2, is formed instead of the long-chain metaphosphate. The acid diphosphates Mg2(H2P2O7)2 and Zn2(H2P2O7)2 reported in the powder diffraction file are isotypic to Ni2(H2P2O7)2.
