Solving a puzzle: chemical and structural characterization of bismuth ammonium oxalate hydrate

Vanhoyland G.^a, Le Bail A.^b, Mullens J.^a and Van Poucke L.C.^a

^a Laboratory for Inorganic and Physical Chemistry, IMO, Limburgs Universitair Centrum, Wetenschapspark 1, 3590 Diepenbeek (Belgium)

^b Laboratoire des Fluorures, Université du Maine, 72085 Le Mans (France)

As part of a systematic study of the Bi-Sr-Ca-Cu system, single and multi metal oxalate compounds were synthesized. For the monometallic bismuth oxalate, the well known Bi₂(C₂O₄)₃×7H₂O and Bi₂(C₂O₄)₃ were obtained, except for very low Bi concentration. Under these particular conditions a crystalline compound was obtained with a tetragonal unit cell. Before any attempt could be made to solve the crystal structure of this unknown compound, a thorough chemical characterization was necessary. However, from conventional TGA measurements alone no sound conclusion could be made with regard to the stoicheometry. By combining information obtained from several hyphenated thermal analysis techniques, infrared and a quantitative chemical analysis, Bi(NH₄)(C₂O₄)₂×3.71H₂O was then deduced to be the correct chemical formula. From the structural analysis, the reversible water exchange, as observed with HT-XRD and TGA, could be explained by the trapping of water molecules in the open bismuth-oxalate network. In contrast to the ammonium ions, these water molecules are not connected to the network by means of hydrogen bridges, which explains their high mobility.