Structure determination from powder diffraction data and thermal behaviour of layered lead nitrate oxalate hydrate, Pb2(NO3)2(C2O4).2H2O

The mixed lead nitrate oxalate, Pb-2(NO3)(2)(C2O4)(.)2H(2)O, has been obtained in a polycrystalline form in the course of a study on precursors of nanocrystalline PZT-type oxides, Its crystal structure has been solved from powder diffraction data collected using a monochromatic radiation from a conventional X-ray source. The symmetry is monoclinic, space group P2(1)/c (No. 14), the cell dimensions are a = 10.623(2) Angstrom, b = 7.9559(9) Angstrom, c = 6.1932(5) Angstrom, beta = 104.49(1)degrees and Z = 4. The structure consists of a stacking of complex double sheets parallel to (100), forming layers held together by hydrogen bonds. The sheets result from the condensation of PbO10 polyhedra, in which the oxalate and nitrate groups, as well as water molecules, play a major role. The structure is discussed in terms of Pb-O distances, polyhedra shape and lead coordination, with emphasis on the dimensional polymerisation role of water molecules. The thermal behaviour of this layered compound is carefully described from temperature-dependent powder diffraction and thermogravimetric measurements. The enthalpy, Delta(r)H = 232(3) kJ mol(-1), and entropy, Delta(r)S = 532(8) J K-1 mol(-1), of the dehydration reaction have been determined. The high value Of Delta(r)H demonstrates that the water molecules are strongly bonded in the structure. The complex decomposition proceeds through the crystallisation and decomposition of Pb(NO3)(2)(C2O4) into Pb(NO3)(2) and PbC2O4, and, finally, various lead oxides. (C) 2001 Editions scientifiques et medicales Elsevier SAS. All rights reserved.