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

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 Au_x ion peaks to calibrate high masses. For UO₂ surfaces, the dominant U ₃O_x and U₄O_y cations were U ₃O₆⁺ and U₄O₈ ⁺, whereas for both U₃O₈ and UO₃ surfaces, they were U₃O₇⁺ and U ₄O₉⁺. 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 U ₃O₈ 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.