THERMAL-DECOMPOSITION OF NOVEL RARE-EARTH SELENITES

TG and DTA have been carried out on new anhydrous rare-earth selenites R2SeaO3+2a (a = 3.5, 4) in order to establish their stability. Decomposition occurs in three steps attributed to successive losses of SeO2. The first process gives rise to other new group of selenites of composition R2Se3O9, which crystallize in two different forms depending on the rare-earth element. The second process leads to isomorphous compounds R2SeO5. The final product of thermal degradation is R2O3. All products were characterized by chemical analysis and X-ray powder diffraction methods.