Synthesis, characterization and the thermal decomposition of lithium tris(oxalato)lanthanum(III)nonahydrate and sodium tris(oxalato)lanthanum(III)octahydrate

Lithium tris(oxalato)lanthanum(III)nonahydrate (Li-3[La(III)(C2O4)(3). 9H(2)O(LOL)), and sodium tris(oxalato)lanthanum(III)octahydrate (Na-3[La(III)(C2O4)(3)]. 8H(2)O (SOL)), have been synthesized and characterized by elemental analysis, IR spectral, and X-ray powder diffraction studies. Thermal studies (TG, DTG and DTA) in air showed that LOL becomes anhydrous around 198 degrees C, followed by decomposition to Li2CO3, La2O3 and LiLaO2 at ca. 542 degrees C, via the formation of several intermediates. Finally, around 950 degrees C, the oxide and peroxide of lithium and a trace of La2C3 along with La2O3 and LiLaO2 are detected. A DSC study in nitrogen suggested the presence of lithium oxalate and La2O3 around 660 degrees C, The compound, SOL, lost all eight water molecules within 230 degrees C, followed by decomposition to Na2CO3 and La2O(CO3)(2) at ca. 538 degrees C. The pyrolyzed product at 950 degrees C contained Na2CO3 (trace), Na2O, Na2O2 (trace), La2O3, probably also NaLaOx. The residual product in nitrogen at 630 degrees C is assumed to be a mixture of Na2CO3, NaO2 and La2O3. The kinetic parameters of dehydration and decomposition have been calculated from TG curves using four non-mechanistic equations. Kinetic parameters E*, Delta H, Delta S obtained from DSC study are discussed. Some of the decomposition products were identified by IR and X-ray powder diffraction studies. The tentative mechanism for the decomposition in air are proposed. (C) 1999 Elsevier Science B.V. All rights reserved.