163 research outputs found
Transmutation in 90SrF2: A density functional theory study of phase stability in ZrF2
The stability of multiple possible phases of ZrF2 is computed using density-functional theory. Motivated by radioactive samples of fluorite 90SrF2 stored at the Hanford site, we consider β− radioactive decay as the route by which the 90ZrF2 is generated. To find suitable structures for the ZrF2 compound two methodologies are used. The first follows imaginary phonon modes from the fluorite ZrF2 while the second employs random structure searching. Six possible ZrF2 phases are identified; however, none of the structures resemble the lone experimentally reported orthorhombic structure for ZrF2. Although we predict these phases to be less stable (~0.3 eV/f.u.) than a phase-decomposed mixture of β-ZrF4 and Zr metal, they still may be relevant due to the kinetics of formation via radioactive decay and raise questions as to the nature of the ZrF2 structure and the state of the samples at Hanford
Hydrogen Bond Disruption in DNA Base Pairs from 14C Transmutation
Recent ab initio molecular dynamics simulations have shown that radioactive carbon does not normally fragment DNA bases when it decays. Motivated by this finding, density functional theory and Bader analysis have been used to quantify the effect of C → N transmutation on hydrogen bonding in DNA base pairs. We find that 14C decay has the potential to significantly alter hydrogen bonds in a variety of ways including direct proton shuttling (thymine and cytosine), thermally activated proton shuttling (guanine), and hydrogen bond breaking (cytosine). Transmutation substantially modifies both the absolute and relative strengths of the hydrogen bonding pattern, and in two instances (adenine and cytosine), the density at the critical point indicates development of mild covalent character. Since hydrogen bonding is an important component of Watson-Crick pairing, these 14C-induced modifications, while infrequent, may trigger errors in DNA transcription and replication
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Nanoscale oxygen defect gradients in UO2+x surfaces.
Oxygen defects govern the behavior of a range of materials spanning catalysis, quantum computing, and nuclear energy. Understanding and controlling these defects is particularly important for the safe use, storage, and disposal of actinide oxides in the nuclear fuel cycle, since their oxidation state influences fuel lifetimes, stability, and the contamination of groundwater. However, poorly understood nanoscale fluctuations in these systems can lead to significant deviations from bulk oxidation behavior. Here we describe the use of aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy to resolve changes in the local oxygen defect environment in [Formula: see text] surfaces. We observe large image contrast and spectral changes that reflect the presence of sizable gradients in interstitial oxygen content at the nanoscale, which we quantify through first-principles calculations and image simulations. These findings reveal an unprecedented level of excess oxygen incorporated in a complex near-surface spatial distribution, offering additional insight into defect formation pathways and kinetics during [Formula: see text] surface oxidation
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Nanoscale oxygen defect gradients in UO<sub>2+x</sub> surfaces
Oxygen defects govern the behavior of a range of materials spanning catalysis, quantum computing, and nuclear energy. Understanding and controlling these defects is particularly important for the safe use, storage, and disposal of actinide oxides in the nuclear fuel cycle, since their oxidation state influences fuel lifetimes, stability, and the contamination of groundwater. However, poorly understood nanoscale fluctuations in these systems can lead to significant deviations from bulk oxidation behavior. Here we describe the use of aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy to resolve changes in the local oxygen defect environment in </p
Nanoscale Oxygen Defect Gradients in the Actinide Oxides
Oxygen defects govern the behavior of a range of materials spanning
catalysis, quantum computing, and nuclear energy. Understanding and controlling
these defects is particularly important for the safe use, storage, and disposal
of actinide oxides in the nuclear fuel cycle, since their oxidation state
influences fuel lifetimes, stability, and the contamination of groundwater.
However, poorly understood nanoscale fluctuations in these systems can lead to
significant deviations from bulk oxidation behavior. Here we describe the first
use of aberration-corrected scanning transmission electron microscopy and
electron energy loss spectroscopy to resolve changes in the local oxygen defect
environment in UO surfaces. We observe large image contrast and spectral
changes that reflect the presence of sizable gradients in interstitial oxygen
content at the nanoscale, which we quantify through first principles
calculations and image simulations. These findings reveal an unprecedented
level of excess oxygen incorporated in a complex near-surface spatial
distribution, offering new insight into defect formation pathways and kinetics
during UO oxidation.Comment: 26 pages, 12 figure
Thickness Dependent OER Electrocatalysis of Epitaxial LaFeO Thin Films
Transition metal oxides have long been an area of interest for water
electrocatalysis through the oxygen evolution and oxygen reduction reactions.
Iron oxides, such as LaFeO, are particularly promising due to the
favorable energy alignment of the valence and conduction bands comprised of
Fe cations and the visible light band gap of such materials. In this
work, we examine the role of band alignment on the electrocatalytic oxygen
evolution reaction (OER) in the intrinsic semiconductor LaFeO by growing
epitaxial films of varying thicknesses on Nb-doped SrTiO. Using cyclic
voltammetry and electrochemical impedance spectroscopy, we find that there is a
strong thickness dependence on the efficiency of electrocatalysis for OER.
These measurements are understood based on interfacial band alignment in the
system as confirmed by layer-resolved electron energy loss spectroscopy and
electrochemical Mott-Schottky measurements. Our results demonstrate the
importance of band engineering for the rational design of thin film
electrocatalysts for renewable energy sources.Comment: 19 pages, 6 figures; authors Burton and Paudel contributed equally;
supplement: 11 pages, 7 figure
Percolation of Ion-Irradiation-Induced Disorder in Complex Oxide Interfaces
Mastery of order-disorder processes in highly non-equilibrium nanostructured
oxides has significant implications for the development of emerging energy
technologies. However, we are presently limited in our ability to quantify and
harness these processes at high spatial, chemical, and temporal resolution,
particularly in extreme environments. Here we describe the percolation of
disorder at the model oxide interface LaMnO / SrTiO, which we visualize
during in situ ion irradiation in the transmission electron microscope. We
observe the formation of a network of disorder during the initial stages of ion
irradiation and track the global progression of the system to full disorder. We
couple these measurements with detailed structural and chemical probes,
examining possible underlying defect mechanisms responsible for this unique
percolative behavior.Comment: 32 pages, 14 figure
Resolving diverse oxygen transport pathways across Sr-doped lanthanum ferrite and metal-perovskite heterostructures
Perovskite structured transition metal oxides are important technological
materials for catalysis and solid oxide fuel cell applications. Their
functionality often depends on oxygen diffusivity and mobility through complex
oxide heterostructures, which can be significantly impacted by structural and
chemical modifications, such as doping. Further, when utilized within
electrochemical cells, interfacial reactions with other components (e.g. Ni-
and Cr-based alloy electrodes and interconnects) can influence the perovskite's
reactivity and ion transport, leading to complex dependencies that are
difficult to control in real-world environments. Here we use isotopic tracers
and atom probe tomography to directly visualize oxygen diffusion and transport
pathways across perovskite and metal-perovskite heterostructures, i.e. (Ni-Cr
coated) Sr-doped lanthanum ferrite (LSFO). Annealing in 18O2(g) results in
elemental and isotopic redistributions through oxygen exchange (OE) in the LSFO
while Ni-Cr undergoes oxidation via multiple mechanisms and transport pathways.
Complementary density functional theory (DFT) calculations at experimental
conditions provide rationale for OE reaction mechanisms and reveal a complex
interplay of different thermodynamic and kinetic drivers. Our results shed
light on the fundamental coupling of defects and oxygen transport in an
important class of catalytic materials.Comment: 39 pages, 10 figure
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