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

Aqueous solutions containing the nanoscale uranyl peroxide cage clusters U₆₀, [(UO₂)(O₂)(OH)]₆₀^60-, and U₆₀Ox₃₀, [{(UO₂)(O₂)}₆₀(C₂O₄)₃₀]^60-, were monitored by in situ Raman spectroscopy during stepwise heating to 180 °C. In solutions containing U₆₀, clusters persist to 120 °C, although conversion of U₆₀ to U₂₄, [(UO₂)(O₂)(OH)]₂₄^24-, occurs above 100 °C. U₆₀Ox₃₀ 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, K₂[(UO₂)₃O₂(OH)₃]₂(H₂O)₇, and metaschoepite, [(UO₂)₈O₂(OH)₁₂](H₂O)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.