Crystal structure and thermal behavior of La(H2O)2M(C2O4)2•H2O (M = K, NH4) studied by powder X-ray diffraction

The crystal structure of La(H2O)(2)K(C2O4)(2). H2O has been solved ab initio from powder diffraction data collected with conventional monochromatic S-rays, The symmetry is monoclinic, space group C2/m, cell dimensions a 22.033(1) Angstrom, b = 7.6003(5) Angstrom, c = 6.6418(4) Angstrom, beta = 103.813(6)degrees, V = 1080.05(8) Angstrom(3), Z = 4. Since a disorder of one oxalate group is observed with all possible monoclinic space groups, it has been found convenient to describe the crystal structure in a triclinic subcell, The disorder problem is thoroughly discussed with respect to the powder diffraction data available for this material, From this description the compound exhibits a layered-type structure, which could explain reported cation-exchange properties. The layers are formed by four-membered rings [La(C2O4)](4). The potassium atom and two of the three water molecules are located between the layers. The third water molecule lies on the layer in tunnels with a diamond-shaped cross section. The lanthanum atom is tenfold-coordinated in a distorted bicapped square antiprism and the potassium atom is eightfold-coordinated in a dodecahedron, The crystal structure of La(H2O)(2)(NH4)(C2O4)(2). H2O is isostructural with that of La(H2O)(2)K(C2O4)(2). H2O [a = 22.130(4) Angstrom, b = 7.774(1) Angstrom, c = 6.655(2) Angstrom, beta = 105.28(2)degrees, V = 1104.4(3) Angstrom(3)]. The thermal decomposition of these two precursors has been carried out by means of temperature-dependent X-ray diffraction and TGDSC, The dehydration leads to anhydrous phases, which are amorphous and poorly crystalline for the potassium and ammonium compounds, respectively. Amorphous LaK(C2O4)(2) crystallizes at 260 degrees C. Its symmetry is monoclinic with the cell dimensions a = 5.687(3) Angstrom, b = 15.241(5) Angstrom, c = 9.017(4) Angstrom, beta = 92.82(3)degrees. At higher temperature, the decomposition of LaK(C2O4)(2) yields La2O2CO3 (Type-Ia), La2O2CO3 (Type-II), and pure La2O3, successively, La(NH4)(C2O4)(2) leads to La2O2CO3 (Type-II) and, finally, La2O3. (C) 2000 Academic Press.