Pu-Ga Research Articles

A first-principles study on Ga stabilized δ-Pu phase stability based on structural and electronic properties

The high temperature face-centered-cubic delta phase of plutonium (FCC δ-Pu) is metallurgically important in industrial applications and can be stabilized at room temperature by alloying with small amounts of gallium (Ga). Therefore, it is of fundamental interest to elucidate the factors that govern the δ-Pu-Ga interactions via the electronic structure to understand the stabilization mechanism of the FCC phase. We employed density functional theory to systematically model the change in the structural and electronic properties with increasing Ga concentrations of δ-Pu-Ga alloys. The results indicate that structural optimizations with an applied cubic symmetry constraint are sufficient to describe the alloy properties. The variations in Pu-Pu and Pu-Ga bond lengths and the lattice disorder induced by the Ga substitution at Pu sites are in qualitative agreement with previously published experimental EXAFS data. Additionally, in accordance with the experimental results, the atomic volume decreases as the Ga concentration increases. The alloyformation energydecreasesas the Ga concentrationincreases, indicating an increase in stability and an exothermic alloying reaction. The Pu-Ga electronic interaction is governed principally by hybridization between the Pu 6d and Ga 4p states, with decreasing Pu 5f states at the Fermi energy with increasing Ga content due to the increase of Ga-Pu-Ga bonds within the lattice.

First-Principles Study on the Adsorption Behavior of O2 on the Surface of Plutonium Gallium System.

To deeply understand the adsorption process of oxygen on the surface of a plutonium gallium system and to reveal the chemical reaction mechanism at the initial stage of oxidative corrosion on the surface of plutonium gallium alloy at a theoretical level, the adsorption behavior of oxygen molecules on the surface of a plutonium gallium system was investigated by a first-principles approach based on density flooding theory. The results show that the molecular bond length increases and finally breaks when the surface oxygen molecule is adsorbed on the surface of plutonium gallium system and dissociates into two atomic states. The most likely adsorption position of oxygen molecules on the surface of plutonium gallium system is hole-site vertical adsorption with the adsorption energy size of 10.7 eV. The bonding between oxygen atom and surface is mainly due to the overlapping hybridization of Pu-6s, Pu-7s, Pu-6d, Ga-3d and O-2p orbitals. Oxygen molecules mainly interact with the atoms of the first layer on the surface of the plutonium gallium system. The oxygen atoms after stable adsorption are able to diffuse to the subsurface of the plutonium gallium system after overcoming the energy barrier of 16.7 eV and form a stable structure. The research results reveal the initial reaction process and adsorption law of oxygen on the surface of plutonium gallium system from microscopic level, which is helpful to further explore the surface corrosion prevention technology of plutonium gallium system and improve the reliability and safety of plutonium gallium alloy components.

Evidence of an oxidation induced phase transformation for a delta phase plutonium-gallium alloy

The oxidation of an ~1.5 at% Ga plutonium metal alloy in dry air as a function of time was studied using a combination of FIB-SEM, AES, and XRD. Analysis of SEM images of FIB cross-sections of the oxide allowed for direct measurement of the oxide thickness as well as determination of heterogeneity in the extent of the oxide. AES analysis performed on cross-sections of the oxide allowed for quantitative analysis of the atomic composition and semi-quantitative analysis of the chemical composition. Initial preparation of the sample was found to induce a martensitic transformation to α`-Pu with an average thickness of 233 nm. Electropolishing was found to be sufficient to fully remove the α` layer with creation of only a minimum oxide layer. Growth of the oxide in dry air was found to initially follow parabolic kinetics, with a parabolic rate constant orders of magnitude higher than expected for δ-Pu but in-line with values extrapolated to room temperature for α-Pu. Following complete oxidation of the initial α` layer, further regions of plutonium metal depleted in gallium were observed to form at the oxide / metal interface. However, gallium, as Ga2O3, was found to be enriched at the surface and present in the oxide in an amount comparable to the relative Ga:Pu ratio of the bulk metal alloy. The results presented here serve to bridge a diverse array of previous experimental and theoretical data and provide the first comprehensive view of the fate of the δ-stabilizer during formation of an oxide. Data availabilityThe raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Inverse photoemission of the light actinide metals and dioxides

Inverse photoemission spectroscopy (IPES) operating in the isochomat mode (≈10 eV) has been used to probe the unoccupied states in the actinide dioxides of thorium, uranium and plutonium, and the actinide metals of thorium, uranium, α-plutonium and a plutonium-gallium alloy, δ-Pu(Ga). The IPE spectra of the dioxides show a signal predominantly occurring from the 5f unoccupied states and where possible have been curve fit to obtain the 5f5/2 – 5f7/2 spin orbit coupling and intensity values. Combined with the HeI Ultraviolet Photoelectron Spectroscopy of the actinide dioxides the band gap values have been determined. The IPE spectra of the metals show a weak signal on a large background contribution with little difference between the α- and δ-Pu(Ga) phases. The IPE spectra of the metals have been used to aid interpretation of the possible screening mechanisms in the 4f core level photoelectron spectra. Photoelectric work functions for these materials are also reported. UK Ministry of Defence © Crown Owned Copyright 2021/AWE

Long-term behavior of vacancy defects in Pu-Ga alloy: Effects of temperature and Ga concentration

Vacancy and its clusters are among the most important defects induced by self-irradiation in plutonium-gallium (Pu-Ga) alloys. For decades-long stockpiles, the generation and evolution of vacancy defects could cause void swelling and helium bubble formation, resulting in the ageing of the Pu-Ga alloys. Therefore, to shed light on the ageing mechanisms caused by vacancies in Pu-Ga alloys, the long-term behaviour of vacancy defects in Pu-Ga alloys is simulated employing an AKMC model parameterized by molecular statics calculations. By tracking the number of vacancy defects, the size distribution of vacancy clusters, and the largest vacancy clusters over time, we found that temperature and Ga concentration significantly influence the evolution of vacancy defects in Pu-Ga alloys. Temperature changes could affect the clustering behaviour of mono vacancies, and critical temperatures initializing the nucleation of vacancy clusters are observed. On the other hand, adding Ga contents in Pu-Ga alloys could increase the migration energy of and attractive interaction among vacancies, leading to increased vacancy cluster numbers at high temperatures.

Magnetic order and valence fluctuation in a PuGa intermetallic compound studied via a first principles calculation

AbstractIn order to reveal electronic properties of a plutonium‐gallium intermetallic compound (Pu3Ga), and its potential implication for microscopic mechanisms for effects of Ga doping on the electronic and structural properties, as well as the phase stability of delta‐phase PuGa alloy, a first principles calculation on the magnetic properties of this system is implemented by using density functional theory (DFT) plus on‐site Coulomb repulsion U with nonmagnetic, ferromagnetic, and antiferromagnetic (AFM) orders, while the intermediate correlation effect, which is beyond the scope of pure itinerant and localized electronic model, is investigated by using a many‐body technique combining DFT and dynamical mean‐field theory considering the dynamical correlation effect due to the incompletely filled Pu 5f orbitals and the relativistic effect by inclusion of spin‐orbit coupling (SOC). Our findings show that Pu3Ga is a bad metal with AFM order, which is in good agreement with the experimental magnetic measurement. SOC further splitting Pu 5f states into j = 5/2 and j = 7/2 manifolds, the former exhibits metallic character, while the latter insulating feature. Occupation analysis establishes that an average occupancy of Pu 5f electrons in Pu3Ga is nf = 4.9598, this result together with the spectrum function indicates that 5f electrons in this system might be a localized state with strong valence fluctuation. Additionally, optimization of lattice parameter, density of state, and momentum‐resolved electronic spectrum function are also presented.

Energetics and diffusion of point defects in Au/Ag metals: A molecular dynamics study**Project supported by the National Natural Science Foundation of China (Grant Nos. 51401237, 11474358, and 51271198), the Fund from Shaanxi Provincial Education Department, China (Grant No. 18JK1207), and the Defence Technology Foundation of China (Grant No. 2301003).

To reveal the potential aging mechanism for self-irradiation in Pu–Ga alloy, we choose Au–Ag alloy as its substitutional material in terms of its mass density and lattice structure. As a first step for understanding the microscopic behavior of point defects in Au–Ag alloy, we perform a molecular dynamics (MD) simulation on energetics and diffusion of point defects in Au and Ag metal. Our results indicate that the octahedral self-interstitial atom (SIA) is more stable than the tetrahedral SIA. The stability sequence of point defects for He atom in Au/Ag is: substitutional site octahedral interstitial site tetrahedral interstitial site. The He–V cluster (, V denotes vacancy) is the most stable at n = m. For the mono-vacancy diffusion, the MD calculation shows that the first nearest neighbour (1NN) site is the most favorable site on the basis of the nudged elastic band (NEB) calculation, which is in agreement with previous experimental data. There are two peaks for the second nearest neighbour (2NN) and the third nearest neighbour (3NN) diffusion curve in octahedral interstitial site for He atom, indicating that the 2NN and 3NN diffusion for octahedral SIA would undergo an intermediate defect structure similar to the 1NN site. The 3NN diffusion for the tetrahedral SIA and He atom would undergo an intermediate site in analogy to its initial structure. For diffusion of point defects, the vacancy, SIA, He atom and He–V cluster may have an analogous effect on the diffusion velocity in Ag.

A molecular dynamics simulation of energetics and diffusion of point defects in a Au–Ag alloy

For revealing an aging mechanism for self-irradiation in a Pu–Ga alloy, we carried out a molecular dynamics (MD) simulation on a substitutional material, i.e., Au–Ag alloy. In this work, we estimate physical and microscopic properties of the Au–Ag alloy containing various point defects using a MD method, in particular, formation energy for point defects, migration energy for point defects diffusion into interstitial sites, and diffusion coefficient for the Au–Ag alloy containing point defects, such as vacancy, He atom and He-vacancy (He-V) cluster. The results indicate that volumetric heat capacity and linear expansion coefficient would decrease due to the various point defects, and He atom has the most remarkable influence on the physical properties of the Au–Ag alloy for point defects considered in this work. The formation energy of Au and Ag self-interstitial atom indicates that Octa1 is the most stable site, and structural stability of octahedral (Octa) interstitial sites for the He atom obeys \(\hbox {Octa1}> \hbox {Octa2}> \hbox {Octa4} > \hbox {Octa3}\). For the \(\hbox {He}_{n}\hbox {V}_{m}\) cluster, the formation energy of the defect structure is most stable at \(n = m\). The diffusion coefficient of the He-V cluster is relatively smaller, showing that vacancy defects would further decrease atomic diffusion. An influence of various point defects on the diffusion velocity in the Au–Ag alloy obeys the He-V \(\hbox {cluster}> \hbox {He}> \hbox {vacancy}> \hbox {Ag} > \hbox {Au}\).

Influence of Ga on H Reactivity with Ga-Stabilized δ-Pu Alloys

The influence of Ga on the adsorption and solubility of H on Ga-stabilized δ-Pu alloys is investigated using density functional theory and experimentally via a Sieverts’ apparatus. All-electron DFT calculations, with spin–orbit coupling and full relaxations of the surface, are implemented to calculate H adsorption on δ-Pu–Ga alloy (111) surfaces. Chemisorption energies indicate that H prefers a 3-fold hollow site that binds strongly to a Pu-rich environment regardless of Ga concentration of the surface. Sites that include a Ga–H bond yield an increase in chemisorption energies as opposed to chemisorption energies when the H is only bonded to Pu atoms. Also, solution energies for interstitial sites are less exothermic than surface adsorption energies. Experimentally, H-vacancy binding is more exothermic than interstitial binding, and since vacancy binding is the analogue of surface binding, the DFT results are in agreement. Both DFT and experimental results show that with increasing Ga content to 7 at. % Ga there is an increase in H solution energies, thus reducing H solubility. The maximal reduction of H solubility is between 4 and 5 at. % Ga stabilized δ-Pu. The bonding nature of H on the Pu–Ga surface, via analysis of the partial density of states, shows weak hybridizations between the Pu 6d and H 1s states, with a slight decrease of the Pu 5f states near the Fermi level. Overall, Ga will affect the surface adsorption and solubility of H in δ-Pu–Ga alloys.

Influence of point defects and impurities on the dynamical stability of δ-plutonium

We use first-principles calculations to provide direct evidence of the effect of aluminum, gallium, iron and uranium on the dynamical stability of δ-plutonium. We first show that the δ phase is dynamically unstable at low temperature, as seen in experiments, and that this stability directly depends on the plutonium 5f orbital occupancies. Then, we demonstrate that both aluminum and gallium stabilize the δ phase, contrary to iron. As for uranium, which is created during self-irradiation and whose effect on plutonium has yet to be understood, we show that it leaves a few unstable vibrational modes and that higher concentrations lead to an almost complete stabilization. Finally, we provide an attempt at a consistent analysis of the experimental Pu–Ga phonon density of states. We show that the presence of gallium can reproduce only partially the experimental measurements, and we investigate how point defects, such as interstitials and vacancies, affect the calculated phonon density of states.

Quantitative method for describing heterogeneity in plutonium-gallium alloys

A new method for describing gallium (Ga) heterogeneity in plutonium–gallium (Pu–Ga) alloys in terms of two parameters, dx and m0, is presented in this paper. The parameters are derived from a modified Boltzmann function. The resulting sigmoidal curve function is fitted to a grain radius bearing curve, derived from quantitative Ga distribution map data. Describing the heterogeneity in terms of the derived parameters enables the effect of different parameters on the rate of homogenisation of the Ga to be quantitatively characterised. This quantification will allow us to determine the relationships that material properties and heat treatment conditions have on the rate of homogenisation of the Ga. Preliminary results presented here indicate a strong log–log correlation between the heat treatment time at a given temperature and the resulting heterogeneity, and also indicates that the combined iron (Fe) and nickel (Ni) content in the alloy affects the homogenisation rate.

Thermal cycling as a novel method to experimentally derive the “equilibrium” Pu–Ga phase diagram

The feasibility of using thermal cycling to experimentally derive the “equilibrium” Pu–Ga phase diagram in the Pu-rich end is discussed, in particular the effect of thermal cycling on the decomposition of δ-stabilized Pu alloys at ambient pressure. This is an alternative to the Russian method and was originally proposed by the author on the basis of a thorough analysis of data from a systematic thermal cycling experiment performed on annealed and aged δ-stabilized Pu–Ga alloys. These were subject to repetitive thermal cycling in the temperature range 25–180 °C, which was found to drive and accelerate the Pu–Ga system towards its “equilibrium” state, as demonstrated by continuous α-phase in-growth with each thermal cycle. An experimental programme to confirm these initial observations is proposed, and the “constrained” equilibrium Pu–Ga phase diagram that might be expected from this method is outlined.

Structure and phase stability of a Pu-0.32 wt% Ga alloy

In plutonium-gallium (Pu-Ga) alloys that have a Ga content of 0.3–0.4 wt%, their readiness to transform to α′ renders them of particular interest in efforts to understand the tenuous nature of δ phase stability. The present study is a comprehensive examination of the structure and phase stability of a cast Pu-0.32 wt% Ga alloy, the Ga content being close to the minimum amount needed to retain the δ phase to ambient temperature. The alloy was characterised in both the as-cast condition as well as following a homogenising heat treatment. The 250-h heat treatment at 450 °C was shown to achieve an apparently stable δ-Pu phase. However, the stability of the δ-Pu phase was shown to be marginal: partial transformation to α′-Pu was observed when the alloy was subjected to hydrostatic compression. Similar transformation was also apparent during metallographic preparation as well as during hardness indentation. The results provide new understanding of the nature of δ phase stability.

Equilibrium thermodynamics of radiation defect clusters in δ-phase Pu–Ga alloys

The paper presents a theoretical investigation into the response of δ-phase Pu–Ga alloys to self-irradiation. Using classical molecular dynamics we investigate the long-term behavior of primary radiation defects (vacancies) in the face-centered cubic lattice of the alloys under ambient conditions. High diffusive migration energy barriers and the corresponding low mobility of vacancies do not allow us to track their dynamics in the lattice by direct molecular dynamics simulations. Instead, we use the Helmholtz free energy to investigate the equilibrium thermodynamics of metastable microconfigurations of Pu–Ga crystals with artificially introduced vacancy clusters in various regular and random configurations. The Helmholtz free energy of the microconfigurations are calculated with the thermodynamic integration method. Based on the free energy evaluation we draw conclusions about the relative thermodynamic stability of various microconfigurations under ambient conditions. The equilibrium parameters of vacancy clusters in the bulk of the lattice and in the presence of edge dislocations are estimated.

Understanding oxygen adsorption on 9.375 at. % Ga-stabilized δ-Pu (111) surface: A DFT study

Plutonium (Pu) metal reacts rapidly in the presence of oxygen (O), resulting in an oxide layer that will eventually have an olive green rust appearance over time. Recent experimental work suggested that the incorporation of gallium (Ga) as an alloying impurity to stabilize the highly symmetric high temperature δ-phase lattice may also provide resistance against corrosion/oxidation of plutonium. In this paper, we modeled a 9.375 at. % Ga stabilized δ-Pu (111) surface and investigated adsorption of atomic O using all-electron density functional theory. Key findings revealed that the O bonded strongly to a Pu-rich threefold hollow fcc site with a chemisorption energy of −5.06 eV. Migration of the O atom to a Pu-rich environment was also highly sensitive to the surface chemistry of the Pu–Ga surface; when the initial on-surface O adsorption site included a bond to a nearest neighboring Ga atom, the O atom relaxed to a Ga deficient environment, thus affirming the O preference for Pu. Only one calculated final on-surface O adsorption site included a Ga–O bond, but this chemisorption energy was energetically unfavorable. Chemisorption energies for interstitial adsorption sites that included a Pu or Pu–Ga environment suggested that over-coordination of the O atom was energetically unfavorable as well. Electronic structure properties of the on-surface sites, illustrated by the partial density of states, implied that the Ga 4p states indirectly but strongly influenced the Pu 6d states to hybridize with the O 2p states. The Ga 4p states also weakly influenced the Pu 5f states to hybridize with the O 2p states, even though Ga was not participating in bonding with O.

MD study of the finite temperature effects on the phase ordering, stacking fault energy, and edge dislocation core structure in elemental Pu and Pu–Ga alloys

The Modified Embedded Atom Model (MEAM) of elemental plutonium and plutonium–gallium alloys has been tested for its ability to reproduce the correct ordering of the hcp and fcc phases that is crucial from the point of view of molecular dynamics simulation of elastic–plastic phenomena in the material. Stacking fault energy obtained with the MEAM is in agreement with experimental data. Results of the edge dislocation modeling at the ambient conditions evidence for rather wide dislocation core, namely, 5–6 Burgers vectors. The results of the MD simulation have been compared with those obtained early with recently developed Multi State-MEAM potential.

Phase stability of an aged Pu–0.27 wt.% Ga alloy

This paper describes the characterisation of a naturally aged Pu–0.27wt.% Ga alloy 35years of age. The alloy was subjected to bulk chemical analysis, density determination, differential scanning calorimetry (DSC), optical microscopy, electron probe micro-analysis (EPMA) and hardness measurements. Despite the Ga content being only 0.27wt.%, it nevertheless appears to be sufficient to retain the alloy in the δ-Pu phase at ambient temperature. This was demonstrated by optical microscopy, density measurements and DSC. However, the ambient temperature stability of the δ-Pu phase is tenuous. This was demonstrated by the propensity of the alloy to undergo transformation to α-Pu when cooled to sub-ambient temperatures. Indeed, both density measurements and DSC indicate that when the alloy is cooled to −50°C for 1h the alloy has transformed to more than 40% α-Pu. Moreover, a comparison of the Vickers hardness (expressed as a mean pressure) with transformation pressures for Pu–Ga suggests that the alloy has undergone δ→α′ transformation during the indentation process.

Equilibrium thermodynamics of helium in [formula omitted]-phase Pu–Ga alloys

The paper presents a theoretical study of the response of δ-phase Pu–Ga alloys to self-irradiation. We investigate the long-term behaviour of He atoms in the face-centred cubic (fcc) lattice under ambient conditions by means of classical molecular dynamics. Because of the relatively low diffusive mobility of He atoms in the lattice their dynamics cannot be tracked directly in molecular dynamics simulations. Instead, we use the Helmholtz free energy function to investigate the equilibrium thermodynamics of metastable microconfigurations of perfect crystals with artificially introduced He atoms in different configurations. The Helmholtz free energy of the microconfigurations are calculated using the thermodynamic integration method. Based on the free energy evaluation, inferences about the relative thermodynamic stability of various microconfigurations under ambient conditions are drawn. Estimates of the equilibrium parameters of He bubbles in the lattice and the distribution of He bubble radii are calculated and compared to those observed experimentally.

Thermodynamic re-assessment of the Pu–U system and its application to the ternary Pu–U–Ga system

Phase diagram and thermodynamic properties of the Plutonium–Uranium (Pu–U) system have been successfully re-assessed using the CALPHAD method with input from ab initio electronic-structure calculations for the bcc phase (γ-U, ∊-Pu). Results and methodology are discussed and compared with previous assessments. In addition, the already assessed Pu–Ga (Gallium) and U–Ga data are combined to build the Pu–U–Ga thermodynamic database. The predictions made using this database indicate that a small amount of U impacts the (δ-Pu) Pu–Ga phase stability by precipitating the complicated η and ζ phases that exist in the Pu–U system. Finally, the present investigation provides guidelines for further experimental studies.

Neutron diffraction study of δ-alloy Pu242–Ga aging

In this paper, we report on a continuing neutron diffraction study of the mean-square atom displacements occurring during the long-term self-irradiationof a Pu–Ga alloy. The measurements were performed at room temperature using the sample based on the isotope Pu242 with low neutron absorption cross-section to which the short-lived isotope Pu238 (1.4wt.%) was added to accelerate self-irradiation. We obtain the maximum self-irradiation equivalent time of 35.5years, 12years longer than in our previous papers. In the entire range of self-irradiation time a single fcc phase is preserved. It was found that after the two stages of change in the mean-square displacements we observed earlier (rapid growth up to ∼5–6 equivalent years and a slow decline in the range of ∼6–25years), comes a stage of stabilization (after ∼25years). The stabilization can be explained by the emergence of a balance between the formation of point defects and their absorption by helium bubbles and dislocation loops which accumulated over time.