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

Abstract

Lithium tris(oxalato)lanthanum(III)nonahydrate (Li 3[La(III)(C 2O 4) 3·9H 2O(LOL)), and sodium tris(oxalato)lanthanum(III)octahydrate (Na 3[La(III)(C 2O 4) 3]·8H 2O (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°C, followed by decomposition to Li 2CO 3, La 2O 3 and LiLaO 2 at ca. 542°C, via the formation of several intermediates. Finally, around 950°C, the oxide and peroxide of lithium and a trace of La 2C 3 along with La 2O 3 and LiLaO 2 are detected. A DSC study in nitrogen suggested the presence of lithium oxalate and La 2O 3 around 660°C. The compound, SOL, lost all eight water molecules within 230°C, followed by decomposition to Na 2CO 3 and La 2O(CO 3) 2 at ca. 538°C. The pyrolyzed product at 950°C contained Na 2CO 3 (trace), Na 2O, Na 2O 2 (trace), La 2O 3, probably also NaLaO x . The residual product in nitrogen at 630°C is assumed to be a mixture of Na 2CO 3, NaO 2 and La 2O 3. The kinetic parameters of dehydration and decomposition have been calculated from TG curves using four non-mechanistic equations. Kinetic parameters E *, Δ H, Δ 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.