Synthesis and crystal structure of the hydrated magnesium diphosphate Mg2P2O7 center dot 3.5H(2)O and its high temperature variant Mg2P2O7 center dot H2O

The synthesis and crystal structure of a never hydrated magnesium diphosphate and its high temperature Variant are described. Both structures were solved from powder X-ray diffraction data. The room temperature variant with composition Mg2P2O7. 3.5H(2)O crystallises in the monoclinic space group P2(1)/c (No. 14) with a = 10.9317(1), b = 8.05578(9), c = 9.2774(1) Angstrom, beta = 90.201(1)degrees, V = 816.99(2) Angstrom(3) and Z = 4. The structure consists of sheets stacked along [100] which are linked through MgO2(H2O)(4) pillars into a three-dimensional framework with cavities containing water molecules. Within the sheets there are infinite edge-sharing chains of ME octahedra along [010] which are cross linked by P2O74- groups. A high temperature variant exists around 200 degrees C. The crystal structure of this compound with composition Mg2P2O7. H2O was solved and refined in the monoclinic space group C2/c (No. 15) with a = 18.6596(4), b = 7.9769(1), c = 8.9757(2) Angstrom, beta = 107.378(1)degrees, V = 1275.01(4) Angstrom(3), Z = 8. The transformation to Mg2P2O7. H2O involves removal of the water molecules in the cavities and the water molecules of the Mg octahedral pillars in Mg2P2O7. 3.5H(2)O. The sheets in Mg2P2O7. 3.5H(2)O however remain unchanged during the transformation as the water molecule coordinating Mg here is retained. These sheets are linked through tetrahedral MgO4 pillars into a three-dimensional structure containing infinite 10-membered ring channels along [001]. Both compounds have been further characterised by P-31 MAS NMR spectroscopy, thermogravimetric analysis and high temperature powder X-ray diffraction. (C) 2000 Editions scientifiques et medicales Elsevier SAS. All rights reserved.