The crystal structure of La(H 2 O) 2 K(C 2 O 4 ) 2 ' H 2 O has been solved ab initio from powder di4raction data collected with conventional monochromatic X-rays. The symmetry is monoclinic, space group C2/m, cell dimensions a ؍ 22.033(1) A > , b ؍ 7.6003(5) A > , c ؍ 6.6418(4) A > , ؍ 103.814(6)3, V ؍ 1080.05(8) A > 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 di4raction 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(C 2 O 4 )] 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(
The thermal decomposition of these two precursors has been carried out by means of temperature-dependent X-ray di4raction and TG-DSC. The dehydration leads to anhydrous phases, which are amorphous and poorly crystalline for the potassium and ammonium compounds, respectively. Amorphous LaK(C 2 O 4 ) 2 crystallizes at 2603C. Its symmetry is monoclinic with the cell dimensions a ؍ 5.687(3) A > , b ؍ 15.241(5) A > , c ؍ 9.017(4) A > , ؍ 92.82(3)3. At higher temperature, the decomposition of LaK(C 2 O 4 ) 2 yields La 2 O 2 CO 3 (Type-Ia), La 2 O 2 CO 3 (Type-II), and pure La 2 O 3 , successively. La(NH 4 )(C 2 O 4 ) 2 leads to La 2 O 2 CO 3 (Type-II) and, 5nally, La 2 O 3 .