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

La2O3 takes part in a variety of technologically interesting solids, but it has a known tendency to react with water and CO2, 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 La2O3-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-La2O3 until, upon further heating, only A-La2O3 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-La2O2CO3, in turn, decomposes to La2O3 at 700 °C. Structural relations governing phase conversion within hexagonal or orthogonal-like compounds are discussed.