Thermal studies on alkaline earth uranates

Abstract The thermal stabilities of various alkaline earth uranates were investigated by thermogravimetry and X-ray powder diffraction. New uranates with the formal composition A 2 U 3 O 9 (A=Ca, Sr and Ba) having hexagonal unit cell were obtained and characterised. Oxidation of these uranates led to the formation of two new intermediate compounds of composition A 2 U 3 O 10 (A=Ca and Sr). Many of the calcium and strontium uranates were found to be structurally related to fluorite as long as the substitution of uranium by calcium or strontium was limited to 0.33 and the ratio of oxygen to metal was close to 2. Kinetic studies on the oxidation and the reduction of uranates showed that the oxidation kinetics of A 2 U 3 O 9 to A 2 U 3 O 10 (A=Ca, Sr) or A 2 U 3 O 11 (A=Ba) was controlled by nucleation and growth whereas the reduction kinetics of A 2 U 3 O 11 to A 2 U 3 O 9 (A=Ca, Sr and Ba), generally occurring in single step, was controlled by phase boundaries.