THERMAL AND X-RAY STUDIES ON MIXED-METAL OXALATES - (NH4)LN(III)U(IV)(C2O4)(4)CENTER-DOT-8H(2)O (LN = LA, CE, PR, ND TB)

Five mixed metal oxalates of lanthanides and uranium of the formula (N H4)Ln(III)U(IV)(C2O4)(4) . 8H(2)O were prepared and characterised by c hemical, infrared spectroscopy, X-ray diffraction and thermal methods. The X-ray powder diffraction data of all the compounds were indexed o n an orthorhombic unit cell. Thermal decomposition of the compounds pr oceeded with the loss of water molecules below 475 K followed by decom position in the temperature range of 625 to 655 K to give Ln(2)O(2)CO( 3) and UO2+x as intermediate products. The intermediate products react ed above 975 K to form solid solutions of mixed oxides having a fluori te structure. Activation energy values were derived for the dehydratio n of lanthanum and terbium compounds. Lattice parameters of the fluori te phases of the mixed oxides formed by heating the compounds in air, argon and N-2/H-2 have been determined. (C) 1997 Elsevier Science B.V.