Synthesis, open-framework structure and thermal behaviour of ammonium tin oxalate, Sn2(NH4)2(C2O4)3•3H2O

A new mixed ammonium tin oxalate trihydrate, Sn-2(NH4)(2)(C2O4)(3). 3H(2)O, has been prepared from evaporation of a solution of tin and ammonium oxalates. Its crystal structure has been solved from single-crystal diffraction data. The symmetry is orthorhombic, space group Pnma (No. 62), cell dimensions a = 15.1821(5) Angstrom, b = 11.7506(2) Angstrom, c = 10.8342(3) Angstrom, and Z = 4. The structure consists of macroanionic layers built from [Sn(C2O4)(3)](2-) groups. The SnO6 polyhedron can be described as a pseudo pentagonal bipyramid, with the lone pair of electrons presumably occupying one apex. The resulting framework displays holes in which the water molecules and ammonium groups are located. The thermal behaviour of the mixed ammonium tin oxalate has been investigated with temperature-dependent X-ray powder diffraction and conventional thermal analysis. The degradation process has been completely explained, as well as that of oxammite, a phase always obtained in the preparations. The thermal decomposition of oxammite leads to (NH4)(2)C2O4 and a new acid salt, NH4HC2O4 The mixed ammonium tin oxalate decomposes successively into the amorphous compounds, Sn-2(NH4)(2)(C2O4)(3).H2O and Sn-2(NH4)(2)(C2O4)(3), SnC2O4 and, finally, cassiterite SnO2. (C) 2001 Editions scientifiques et medicales Elsevier SAS. All rights reserved.