Comparative Studies of α- & δ- Plutonium Allotropes & Plutonium Oxidation Mechanisms by Photoemission Spectroscopy

White, Keith

Comparative Studies of α- & δ- Plutonium Allotropes & Plutonium Oxidation Mechanisms by Photoemission Spectroscopy

Open Access

Author:: White, Keith
Graduate Program:: Materials Science and Engineering
Degree:: Master of Science
Document Type:: Master Thesis
Date of Defense:: February 12, 2021
Committee Members:: Patrick M Lenahan, Thesis Advisor/Co-Advisor
Roman Engel-Herbert, Committee Member
Amanda M. Johnsen, Committee Member
John C Mauro, Program Head/Chair
Keywords:: XPS
photoemission
surface science
plutonium
surface
X-ray photoelectron spectroscopy
Auger
f-electron
actinide
oxide
Abstract::      This work investigates the surface oxidation behavior of plutonium (Pu) metal allotropes, α-Pu & gallium-stabilized δ-Pu, as well as epitaxially stabilized polymer assisted deposition (PAD) PuO2 films. The purpose is to evaluate differences in the core level electronic structure between Pu allotropes and Pu-Ga alloys. Photoemission spectroscopy was performed under ultra-high vacuum (UHV) conditions to identify chemical shifts in the Pu 4f core levels. The primary analytical surface science technique employed in this study is X-ray photoelectron spectroscopy (XPS), also known as electron spectroscopy for chemical analysis (ESCA). Secondary techniques used within this work include Auger electron spectroscopy (AES), scanning electron microscopy (SEM), and scanning X-ray imaging (SXI). Shifts in the Pu chemical state are qualitatively evaluated by monitoring Pu 4f core levels. XPS data reduction is performed to quantify the variation in 4f core level lineshape between allotropes and oxidation states.      Experiments are conducted to evaluate the key differences in clean metallic Pu0 4f spectra at the surface of four bulk metal samples: α-Pu, δ-Pu (2 at% Ga), δ-Pu (3 at% Ga), & δ-Pu (7 at% Ga). Pu is known to exhibit a multivalent electron configuration, with two principally observed valence states, a localized [Rn] 5f66d07s2 and itinerant [Rn] 5f56d17s2 valence configuration. Multiple valence structures result in a distinct set of screening potentials between 4f core holes and photo-ejected electrons. This effect manifests as a set of two doublets in the Pu 4f region, termed “localized” and “itinerant” in this work in relation to the valence state responsible for screening. Pu0 4f core level spectra are discussed in detail for each sample, with emphasis on shifts in localized and itinerant components for each Pu-Ga alloy. Screening fractions of 4f electrons are quantified for each Ga alloy composition, acknowledging a trend towards a distribution of predominantly localized valence levels moving from α-Pu to δ-Pu.      Environmental stability of Pu oxides in the solid state holds value for predicting the element’s longevity in a waste storage environment. The ability to understand the formation mechanisms of various solid state oxides on the surface of Pu, and how to functionally control these surface oxides, are important for reactivity in long-duration waste storage environments. In an aqueous solution, there are multiple possibilities for stable complexations with Pu oxidation states, such as Pu3+, Pu4+, Pu5+, & Pu6+. However, in a bulk metallic sample of Pu, the oxidation chemistry is generally reduced to two stable, well-defined, and independently observable solid oxidation states: the sesquioxide Pu3+ (Pu2O3) & the dioxide Pu4+ (PuO2). Applying photoemission spectroscopy techniques, we are able to observe the stability, transformation, and growth of these Pu oxides.       We explore the path forward to functional control of Pu oxides by analyzing the conversion between Pu0(metal), Pu3+, Pu4+, and a possible PuO2+x hyperoxide state on α-Pu, δ-Pu, & PuO2 PAD samples. Samples are sputter cleaned using Ar+ ion bombardment in the range of 2 – 4 keV, with varying sputter spot size settings ranging from 0.25 mm2 to 1 mm2. Conversions of Pu0 to an energetically favored Pu3+ state are observed during long-term UHV containment, offering a passive mechanism for oxide functional control. This mechanism is contrasted to literature in which in-vacuo reduction of Pu4+ to Pu3+ is demonstrated under similar UHV conditions. Preferential sputtering is discussed as a conversion mechanism of the Pu4+ state to Pu3+, applicable for rapid conversion to solid Pu3+ in a research setting. The Pu3+ oxidation state is shown in multiple instances to be the preferred solid oxidation state in a vacuum-environment. Gas dosing experimentation is performed to identify metal-oxide formation for an atmospheric environment. Drierite desiccated local atmosphere is used to perform overnight exposures of the δ-Pu and PuO2 PAD samples. It is demonstrated that desiccated air transforms the sample surface from Pu0 to Pu4+ on a Pu metal substrate. Additionally, we demonstrate the reformation of a preferentially sputtered Pu3+ to Pu4+ on the PuO2 PAD sample by dosing with a 20% O2 – 80% Ar gas mixture. Engineering of a controlled gas dosing reaction cell is discussed in the context of future experimentation of metal-oxide functional control.      PuO2 PAD films grown on yttria-stabilized zirconia (YSZ) substrates are used to confirm the formation and binding energy position of a presumptive PuO2+x hyperoxide peak. Hyperoxide peaks are fitted using oxide lineshape parameters, spin-orbit split values, and an f-orbital branching ratio, conforming to physical parameters of well-observed solid oxidation states. The newly observed hyperoxide doublet is referenced to known metal-oxide 4f7/2 binding energy positions to make an assessment of the possible oxidation state.      Assessments of Pu0 4f core levels for α-Pu and δ-Pu have not been discussed in recent literature. Additionally, there is no available discussion on photoemission trends in Pu0 4f core levels for Pu samples of an increasing alloy constituent, as presented in this thesis. Submissions to peer-reviewed journals will include a qualitative discussion on Pu0 4f core-level screening fractions as a function of bulk Ga atomic concentration. New contributions that will be submitted also include lineshape assessments and experimental discussion on the formation of the PuO2+x hyperoxide state observed on the surface of the PuO2 PAD sample. Work derivative from this thesis report will be submitted, including the acquisition of high resolution α-Pu0 and δ-Pu0 survey spectra for application as a documented reference spectra.

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