Influence of additives on low-temperature hydrothermal synthesis of UO2+x and ThO2

Hydrothermal reactions of uranium(iv) nitrate solutions in the presence of aldehydes yielded microparticles of UO2+x, at temperatures as low as 100 degrees C. The synthesized products contained residual organic matter originating from aldehydes used as an additive, unless the synthesis temperature was 390 degrees C. Additionally, the residual matter could be separated from the product by heat treatment at >300 degrees C. The valence of uranium ions in the UO2+x products was found to be higher than that reported for UO2. Low synthesis temperatures resulted in the suppression of the magnetic moments of uranium ions and the occurrence of the spin-spin interaction among the ions. Therefore, the oxidation number of uranium averaged over the entire product could become higher than 4+. A comparative study with ThO2 suggested that the redox reaction could be closely related to the hydrothermal synthesis reaction and particle growth of UO2+x.