Spectroscopic study of the competition between dehydration and carbonation effects in La2O3-based materials

Abstract La 2 O 3 takes part in a variety of technologically interesting solids, but it has a known tendency to react with water and CO 2 , which may result in nonstoichiometry of the final product and the appearance of impurities. We apply Raman spectroscopy to study hydration and carbonation effects of La 2 O 3 -based materials, using La(OH) 3 as starting material. The thermal evolution of La(OH) 3 reflects the competition between dehydration and carbonation kinetics, and thus is highly dependent upon environment and experimental conditions. In vacuum, La(OH) 3 evolves first to LaOOH and then to a mixture of C- and A-La 2 O 3 until, upon further heating, only A-La 2 O 3 phase remains. In air, La(OH) 3 yields first LaOOH, but under slow heating LaOOH converts to I, Ia and II-type oxycarbonates, in increasing order of thermal stability. II-La 2 O 2 CO 3 , in turn, decomposes to La 2 O 3 at 700 °C. Structural relations governing phase conversion within hexagonal or orthogonal-like compounds are discussed.