Thermal and X-ray studies on mixed alkali uranates of Na-K-U-O system

Solid state reactions of Na2U2O7 and K2U2O7 in air at 1273 K, formed mixed uranates of type (KxNa1−x)U2O7. Reduction of these mixed uranates formed (KxNa1−x)UO3 perovskite type of phase, having cubic structure for 0.5 ≤ x ≤ 1.0 and orthorhombic structure for 0 ≤ x < 0.5. The kinetics of oxidation of (KxNa1−x)UO3 in air, studied by thermogravimetry showed that the reaction is diffusion controlled with an activation energy in the range 135-202 kJ/mol. The enthalpy of oxidation of these perovskite phases was determined from DTA peak area measurement and the values are in the range 40-90 kJ/mol. High temperature X-ray diffraction studies showed that the % thermal expansion increases with increase in sodium concentration in KUO3 phase.