In situ Raman spectroscopy of uranyl peroxide nanoscale cage clusters under hydrothermal conditions

The behaviours of two uranyl peroxide nanoclusters in water heated to 180 °C were examined by in situ Raman spectroscopy. Aqueous solutions containing the nanoscale uranyl peroxide cage clusters U 60 , [(UO 2 )(O 2 )(OH)] 60 60− , and U 60 Ox 30 , [{(UO 2 )(O 2 )} 60 (C 2 O 4 ) 30 ] 60− , were monitored by in situ Raman spectroscopy during stepwise heating to 180 °C. In solutions containing U 60 , clusters persist to 120 °C, although conversion of U 60 to U 24 , [(UO 2 )(O 2 )(OH)] 24 24− , occurs above 100 °C. U 60 Ox 30 persisted in solutions heated to 150 °C, although partial conversion to smaller uranyl peroxide clusters species was observed beginning at 100 °C. Upon breakdown of the uranyl peroxide cage clusters, uranium precipitated as a compreignacite-like phase, K 2 [(UO 2 ) 3 O 2 (OH) 3 ] 2 (H 2 O) 7 , and metaschoepite, [(UO 2 ) 8 O 2 (OH) 12 ](H 2 O) 10 . The role of the countercations, oxalate bridge, and solution pH are examined in order to better understand the mobility of these species at elevated temperatures.