Composition of uranium oxide particles related to TOF‐SIMS ion distributions

Abstract

Uranium oxide particles tens of micrometers in size, including natural and depleted UO2, U3O8, and UO3 were analyzed using an IONTOF V time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) V. Particulate samples were mounted on gold substrates made using a novel technique that reduced hydrocarbon contamination and volatile outgassing as well as provided an internal standard of Aux ion peaks to calibrate high masses. For UO2 surfaces, the dominant U3Ox and U4Oy cations were U3O6+ and U4O8+, whereas for both U3O8 and UO3 surfaces, they were U3O7+ and U4O9+. Secondary ion abundance ratios contained additional information about the chemical composition of the sample related to the relative stability of the cluster ions. Relative stabilities of the most stable cation isomers corresponding to masses observed in SIMS spectra were calculated using high‐level density functional theory in order to compare ion stabilities to TOF‐SIMS intensity distributions. Cation isomers having high oxygen content were doublets, and those having low oxygen content were quartet spin states. Depth profile trends for ‘protonation’ ratio and ‘lattice valence’, as defined by Plog, Wiedmann, and Benninghoven, were used to distinguish U3O8 from UO3. Cations containing a greater number of uranium atoms were also found to have a lower protonation ratio. UO2 and U3O8 surfaces show a steeper reduction in protonation ratio compared to UO3 surfaces which exhibit a nearly constant near‐surface protonation ratio followed by a more gradual smaller decline with depth. We interpret secondary ion distribution results using density functional quantum mechanics calculations comparing the relative stability of cations and anions for different oxygen atom environments. Copyright © 2012 John Wiley & Sons, Ltd.