Structural stability and valence characteristics in cerium hydrothermal systems

Structural stability and valence characteristics in cerium-containing hydrothermal systems were investigated by means of powder X-ray diffraction, infrared, and X-ray photoelectron spectroscopies, Different input species resulted in the stabilization of cerium in different valence states, e.g., direct formation of CeO2 and a novel phase CeOHCO3, isostructural with LaOHCO3. High-temperature calcination of CeOHCO3 led to the formation of CeO2 with high crystallinity and dark brown color. In cerium sulfate systems, both CeO2 and Ce(OH)(3) were found, whereas in nitrate system, ceria with high crystallinity existed as the most stable phase. The mechanisms of the hydrothermal crystallization were discussed and compared with the thermal decomposition reactions of cerium-containing systems. (C) 1996 Academic Press, Inc.