An investigation on the solid-state thermal decomposition behaviour of uranyl(Ⅱ)diaquatris(oxalato)lanthanate(Ⅲ)octahydrate and cobalt(Ⅱ)uranyl(Ⅱ)pentaquatris(oxalato)lanthanate(Ⅲ)octahydrate

摘要

The bimetallic and heterometallic coordination oxalate precursors uranyl(Ⅱ)diaquatris(oxalato)lantha-nate(Ⅲ)octahydrate (UOLa) and cobalt(Ⅱ)uranyl(Ⅱ)pentaquatris(oxalato) Ianthanate(Ⅲ)octahydrate (CoUOLa) have been synthesized and characterized by elemental analysis, IR, electronic spectral and powder X-ray diffraction (XRD) studies. Thermal decomposition studies (TG. DTG and DTA) in air showed that UOLa ultimately decomposed mainly to a mixture of U_2O_5, U_3O_8, La_2O_3 and La_6UO_(12) at 1000 ℃ through the formation of several intermediates at 190,496, and 770 ℃. The presence of a trace of UO_2, carbon and carbides of both uranium and lanthanum were also identified at end products. The precursor, CoUOLa lost all crystal and coordinated water molecules up to 235 ℃, followed by slow decomposition of anhydrous species to a mixture of CoC_2O_4 and (UO_2)_2[La(C_2O_4)_(2.4)]_2 at 330 ℃. The end products identified at 1000 ℃ is a mixture of CoO, LaO and U_2O_5 along with a trace of La_2O_3, LaCoO_3, La_2C_3 and carbon. The unstable intermediate phases at 340 and 515 ℃ were also attributed during the fragmentation path of the precursor. The crystal structures of the major oxides identified in the end products in the mixture form of both the precursors are inferred from the powder XRD data. The kinetic parameters, E°, In k_o, ΔH and ΔS of all the dehydration and decomposition steps of the precursors are explored and discussed from DSC study in nitrogen.