Empirical Potential Calculations Research Articles

Structural and electronic properties of U4O9 from ab initio and empirical potential calculations

We address here the description of the different phases of ${\mathrm{U}}_{4}{\mathrm{O}}_{9}$ with ab initio density functional theory and molecular dynamics empirical potential calculations. These phases are built from the $\mathrm{U}{\mathrm{O}}_{2}$ fluorite by the addition of oxygen interstitials which assemble in clusters named cuboctahedra. Due to the unit cell size and complexity, their simulation represents a formidable task for numerical approaches. With $\mathrm{DFT}+U$, we study in detail two potential structures for $\ensuremath{\alpha}\ensuremath{-}{\mathrm{U}}_{4}{\mathrm{O}}_{9}$, however, these are just two representatives among the numerous coexisting structures we found. The role of the different valence uraniums (${\mathrm{U}}^{4+}, {\mathrm{U}}^{5+}$, and possibly ${\mathrm{U}}^{6+}$) is highlighted thanks to our quantum-mechanical approach. Temperature effects are then appraised with empirical potentials to describe the evolution of the ${\mathrm{U}}_{4}{\mathrm{O}}_{9}$ structure with temperature. We observe a continuous symmetrization of the structure with increasing temperature. Indeed, below 800 K, it gradually turns cubic and the cuboctahedral oxygen clusters tend to symmetrize. Beyond 800 K, the cuboctahedra start to disappear up to 1400 K, where none can be found in our simulated structures.

Atomistic and cluster dynamics modeling of fission gas (Xe) diffusivity in TRISO fuel kernels

TRISO fuel particles are candidates for use in next generation reactors including gas reactors, fluoride salt-cooled high temperature reactors, and micro-reactors. The UCO fuel kernel consists of a uranium dioxide (UO2) and uranium carbide mixture. The addition of UC2 helps suppress the formation of carbon monoxide gas, which led to failures during initial TRISO development. The addition of uranium carbide alters the chemistry of the UO2 kernel, which is known to influence performance parameters such as fission gas diffusivity, although the impact has not been quantified and no models exist that take the change in chemistry into account. Therefore, better understanding and more accurate models of the impact of chemistry on fuel performance are of high priority. In this paper, a first-principles density functional theory (DFT) and empirical potential based multi-scale study has been carried out to model the diffusivity of fission gas xenon (Xe) in UCO TRISO fuel kernels. The focus is on the UO2 component in the UCO fuel kernels, as that represents the largest volume fraction of the fuel kernels. The study relies on DFT and empirical potential calculations to determine Xe and point defect properties, which are then used in thermodynamic and kinetic models to predict diffusion for intrinsic conditions. In addition, the information is utilized in cluster dynamics simulations using the Centipede code to estimate the impact of irradiation on defect transport. The presence of UC2 or UC2−x in the UCO fuel kernels is shown to have a substantial impact on the UO2 non-stoichiometry by inducing oxygen vacancies and driving UO2 sub-stoichiometric, which causes much slower Xe diffusion in UCO compared to light water reactor UO2 fuel. The application of this model in fuel performance simulations using the Bison code is also demonstrated.

Evolving Defect Chemistry of (Pu,Am)O2±x.

The β decay of 241Pu to 241Am results in a significant ingrowth of Am during the interim storage of PuO2. Consequently, the safe storage of the large stockpiles of separated Pu requires an understanding of how this ingrowth affects the chemistry of PuO2. This work combines density functional theory (DFT) defect energies and empirical potential calculations of vibrational entropies to create a point defect model to predict how the defect chemistry of PuO2 evolves due to the incorporation of Am. The model predicts that Am occupies Pu sites in (Pu,Am)O2±x in either the +III or +IV oxidation state. High temperatures, low oxygen-to-metal (O/M) ratios, or low Am concentrations favor Am in the +III oxidation state. Am (+III) exists in (Pu,Am)O2±x as the negatively charged (AmPu1–) defect, requiring charge compensation from holes in the valence band, thereby increasing the conductivity of the material compared to Am-free PuO2. Oxygen vacancies take over as the charge compensation mechanism at low O/M ratios. In (Pu,Am)O2±x, hypo- and (negligible) hyperstoichiometry is found to be provided by the doubly charged oxygen vacancy (VO2+) and singly charged oxygen interstitial (Oi1–), respectively.

A Simple Approach to Growth Mode of InN and InGaN Thin Films on GaN(0001) Substrate

Group-III nitride semiconductors have attracted much attention for application in optoelectronic devices such as light emitting diodes and laser diodes. In particular, highly efficient light emitting devices using self-assembled quantum dots have been pained much attention. For InN and InGaN grown on GaN(0001) substrate, it has been reported that three-dimensional (3D) islands are formed after two-dimensional (2D) wetting layers, resulting in Stranski-Krastanow (SK) growth mode [1,2]. Moreover it has been reported that the growth modes of InN on GaN(0001) substrate during molecular beam epitaxy depend on growth conditions such as V/III ratio and substrate temperature[3]: N-rich condition favors 3D growth, whereas In-rich condition leads to 2D growth. However, it is still unclear what is the crucial factor for determining the growth modes of InN and InGaN depending on the growth condition. In this study, the growth mode of InN and InGaN thin films on GaN(0001) substrate are investigated using the macroscopic theory using free energy formula [4] with the aid of empirical potential calculations [5] as well as ab initio calculations [6]. Effects of pyramids island with {1-103} facets and misfit dislocation (MD) formation are taken into account for the evaluation of free energies of InN and InGaN thin films on GaN(0001) substrate.The calculations of cohesive energy difference ΔE as a function of InN film thickness for 2D coherent growth (2D-coherent) and 2D growth with MD (2D-MD) using empirical interatomic potentials reveals that 2D-MD is more stable than 2D-coherent for InN films beyond 0.7 monolayers (ML). On the basis of these results, the MD formation energy Ed and the effective elastic constant M are estimated to be 1.31 eV/Å and 5.60×1011 N/m2. The calculated result of Ed reasonably agrees with previous calculations for dislocation core energy in bulk InN (1.51 eV/Å) [7]. Moreover, the analysis of free energies using the value of Ed and M and surface energies obtained by ab initio calculations (~ 0.074 eV/ Å2) demonstrate that the critical thickness between growth 2D-coherent and 2D-MD under In-rich conditions is 0.45 ML. The calculated results are qualitatively consistent with the experimental results [3]. The calculated phase diagram for growth mode as functions of surface energy of (0001) and surface energy difference between (0001) and (1-103) planes suggest that growth mode boundary between 2D-MD and 3D coherent growth with hexagonal pyramid island formation (3D-coherent) strongly depends on the surface energy difference between (0001) and (1-103) planes. 2D-MD is favorable when the surface energy of InN(1-103) is larger than that of InN(0001), whereas 3D-coherent is favorable when the surface energies of InN(1-103) is smaller than InN(0001). These results imply that the relative stability between (0001) and (1-103) planes depending on the growth condition is decisive for the growth modes of InN on GaN(0001) substrate.[1] S. Ploch et al., Phys. Status. Solidi C 6, 574 (2009). [2] C. Adelmann et al., Appl. Phys. Lett. 76, 1570 (2000). [3] Y. F. Ng et al., Appl. Phys. Lett. 81, 3960 (2002). [4] K. Shiraishi et al., J. Cryst. Growth 237, 206 (2002). [5] K. E. Khor, and S. Das Sarma, Phys. Rev. B 38, 3318 (1988). [6] T. Akiyama et al., Phys. Status Solidi B 255, 1700329 (2018). [7] T. Ito et al., J. Cryst. Growth 298, 186 (2007).Figure: Phase diagrams of growth modes boundary between 2D-MD and 3D-coherent of InN on GaN(0001) substrate as functions of surface energy of (0001) and surface energy difference between (0001) and (1-103) planes. Figure 1

A Simple Approach to Growth Mode of InN and InGaN Thin Films on GaN(0001) Substrate

The growth of InN and InGaN thin films on GaN(0001) substrates are simply interpreted by using the macroscopic theory using free energy formula with the aid of empirical interatomic potential calculations as well as ab initio calculations. Our empirical interatomic potential calculations reveal that the system with 2D growth with misfit dislocation (2D-MD) is more stable than that with 2D coherent growth (2D-coherent) for InN films beyond 0.7 monolayers (ML), and the thickness for the stabilization of 2D-MD decreases with In composition. Moreover, the analysis of free energies demonstrate that 2D-MD is favored under In-rich conditions while three-dimensional growth with both islands and misfit dislocations is favored under N-rich conditions. The difference in growth mode is due to the surface energy anisotropy depending on the growth condition. The results suggest the surface energy anisotropy is a crucial factor for determining the growth modes of InN and InGaN on GaN(0001) substrate.

A comparative analysis of the phonon properties in UO2 using the Boltzmann transport equation coupled with DFT + U and empirical potentials

The vibrational and thermal properties of UO2 are determined by the solution of the Boltzmann transport equation (BTE) for phonons. The BTE approach relies on interatomic force constants derived from atomic force calculations. Density functional theory (DFT) calculations provide a reliable approach to compute interatomic forces. However, the calculation of interatomic forces using DFT is computationally demanding, and thus significantly limiting its applications. In such cases, empirical potential (EP) calculations are an efficient alternative. However, EPs are not usually constructed to correctly describe lattice vibration properties. In this work, we perform a comparison of lattice vibrations and thermal properties of UO2 solving the BTE using DFT + U and EP computed forces. The experimental lattice thermal conductivity of UO2 is found to be correctly described by DFT + U, while it is only approximated by the Cooper and Potashnikov potentials. However, the heat capacity is only closely reproduced by DFT + U and the Potashnikov potential. In contrast, significant errors were obtained with the Arima and Yamada potentials in all the computed results. The discrepancies in the predicted thermal properties by interatomic potentials were determined to be related to inaccuracies in the description of phonon properties.

Theoretical investigations on the growth mode of GaN thin films on an AlN(0001) substrate

The growth modes of GaN thin films on an AlN(0001) substrate are systematically investigated using our macroscopic theory with the aid of empirical interatomic potential and ab initio calculations. Using empirical interatomic potential calculations, we demonstrate that a GaN/AlN(0001) system with misfit dislocation (MD) is stabilized compared with a coherently grown system when the GaN film thickness exceeds six monolayers. According to the calculated results including the surface energy of GaN, the macroscopic free energy calculations for the growth mode imply that the growth of GaN on AlN(0001) proceeds along the lower-energy path from two-dimensional coherent (2D-coherent) to 2D-MD under Ga-rich conditions but from 2D-coherent to three-dimensional coherent with truncated hexagonal pyramid islands consisting of {} facets under N-rich conditions owing to surface energy anisotropy, which depends strongly on the growth conditions. These results suggest that surface energy anisotropy is crucial for the growth of GaN on an AlN(0001) substrate.

Pyiron: An integrated development environment for computational materials science

To support and accelerate the development of simulation protocols in atomistic modelling, we introduce an integrated development environment (IDE) for computational materials science called pyiron (http://pyiron.org). The pyiron IDE combines a web based source code editor, a job management system for build automation, and a hierarchical data management solution. The core components of the pyiron IDE are pyiron objects based on an abstract class, which links application structures such as atomistic structures, projects, jobs, simulation protocols and computing resources with persistent storage and an interactive user environment. The simulation protocols within the pyiron IDE are constructed using the Python programming language. To highlight key concepts of this tool as well as to demonstrate its ability to simplify the implementation and testing of simulation protocols we discuss two applications. In these examples we show how pyiron supports the whole life cycle of a typical simulation, seamlessly combines ab initio with empirical potential calculations, and how complex feedback loops can be implemented. While originally developed with focus on ab initio thermodynamics simulations, the concepts and implementation of pyiron are general thus allowing to employ it for a wide range of simulation topics.

Local-environment dependence of stacking fault energies in concentrated solid-solution alloys

Concentrated solid-solution alloys (CSAs) based on 3d transition metals have demonstrated extraordinary mechanical properties and radiation resistance associated with their low stacking fault energies (SFEs). Owing to the intrinsic disorder, SFEs in CSAs exhibit distributions depending on local atomic configurations. In this work, the distribution of SFEs in equiatomic CSAs of NiCo, NiFe, and NiCoCr are investigated based on empirical potential and first-principles calculations. We show that the calculated distribution of SFEs in chemically disordered CSAs depends on the stacking fault area using empirical potential calculations. Based on electronic structure calculations, we find that local variations of SFEs in CSAs correlate with the charge density redistribution in the stacking fault region. We further propose a bond breaking and forming model to understand and predict the SFEs in CSAs based on the local structure alone. It is shown that the perturbation induced by a stacking fault is localized in the first-nearest planes for NiCo, but extends up to the third nearest planes for NiFe and NiCoCr because of partially filled d electrons in Fe and Cr.

Influence of vibrational entropy on the concentrations of oxygen interstitial clusters and uranium vacancies in nonstoichiometric UO2

We combine density functional theory (DFT) formation energies and empirical potential calculations of vibrational free energies to calculate the free energies of formation of point defects and clusters of oxygen interstitials, and we use a dilute defect model to calculate the concentrations of defects as a function of temperature and composition. We find that at high temperature oxygen interstitials are dominant, either in isolated form or in clusters depending on the deviation from stoichiometry. At temperatures lower than 1300 K we predict uranium vacancies to be dominant in the stoichiometric material. The disorder in $\mathrm{U}{\mathrm{O}}_{2}$ therefore changes from Schottky to Frenkel type with increasing temperature. Uranium vacancies remain dominant up to deviations form stoichiometry as large as 0.045 at 800 K. Moreover, the concentration of uranium vacancies proves to be non-monotonous as a function of composition. These results are consistent with some experimental data on the evolution with stoichiometry of lattice constant, diffusion coefficients of uranium, positron lifetime and dilatometry measurements.

Theoretical Study of the Electronic Structure of Threading Edge Dislocations in GaN

Although silicon is the material mainly used for power devices, we are now approaching the performance limits of power devices based on silicon. Moreover, it is difficult to improve their performance due to the intrinsic physical properties of silicon. Gallium nitride (GaN), on the other hand, is attracting a lot of attention as an alternative material for future power devices. The bandgap of GaN is three times as wide as that of silicon, and GaN has some superior features, such as its high thermal conductively, large dielectric breakdown field, and high saturation velocity for the electrons. Therefore GaN is expected to become a key material in the next generation of power devices, enabling higher voltages and higher speed operation than current devices. Moreover, the use of GaN will allow miniaturization of the devices and higher temperature operation. During the crystal growth of GaN, however, a high density of threading dislocations appears due to the lattice mismatch with the sapphire (Al2O3), silicon carbide (SiC). Moreover, it has been reported that threading dislocations degrade the performance of GaN-based electronic devices fabricated even on GaN substrates. There have been some experimental studies in which screw or mixed dislocations have been reported as being the primary sources for the leakage current in GaN-based devices [1], whereas threading edge dislocations were found not to contribute to the leakage current [2]. Therefore, we need to fabricate GaN-based power devices to examine the electronic behavior at threading dislocations in GaN and to clarify the origin of the leakage current. As for theoretical studies, the electronic structure of threading dislocations in GaN is not fully understood, although Akiyama et al. reported the electronic structures of threading dislocations by determining the atomistic structures by empirical inter-atomic potential calculations [3]. The purpose of this theoretical study is to discover whether threading edge dislocations contribute to the leakage current or not. To do this, we used first principles calculations based on density functional theory (DFT) to examine the electronic structure at threading edge dislocations with Burghers vectors of 1/3[11-20]. In order to investigate the electronic states of threading edge dislocations in GaN, it was necessary to prepare an initial atomic structure for the dislocations. Our initial atomic structure, which has a supercell containing about 200 atoms, is shown in Fig. 1. Fig. 1(a) and Fig. 1(b) show this model in the [0001] and [1-210] directions, respectively. The model has a periodic boundary condition in the [0001] direction, parallel to the dislocation. The surface of the model shown Fig. 1(a) is terminated by fictitious hydrogen atoms [4] and the model is surrounded by a vacuum region. The shape of the model is like a square pillar. First, the model structure was optimized. It is known that several types of core configurations are proposed for 1/3[11-20] threading dislocations in GaN. In this study we focused on an 8-atom ring core configuration, a symmetric 4-atom ring core configuration, and 5/7-atom ring core configurations. The electronic structures for these relaxed atomic models were calculated. Structural optimization and electronic state calculations were performed using VASP (Vienna Ab-initio Simulation Package) [5], which is a first principles calculation code based on DFT. The electronic densities of states of several core configurations are shown in Fig. 2. Fig. 2(a), Fig. 2(b), Fig. 2(c) and Fig. 2(d) show the electronic densities of states for bulk wurtzite GaN, an 8-atoms ring core, a 5/7-atoms ring core, and a symmetric 4-atoms ring core, respectively. In Fig. 2, the Fermi level is shown by the black lines at 0 eV, and the bandgaps are shown by the shaded blue or green parts. It is known that undoped GaN generally has n-type semiconductor characteristics. Therefore, focusing on the blue parts above the Fermi level, the bandgap of every core configuration is similar to that of bulk GaN. In other words, no defect levels can be seen near the bottom of the conduction band. These results demonstrate that threading edge dislocations in gallium nitride do not contribute to leakage current in GaN-based devices. Moreover, we also discuss the atomic and electronic structures using larger scale systems. Acknowledgement This work was conducted as part of a project entrusted with “research and development for next generation of power devices that contributes to the realization of an energy saving society” (MEXT; Ministry of Education, Culture, Sports, Science and Technology). Figure 1

Grain boundaries in bcc-Fe: a density-functional theory and tight-binding study

Grain boundaries (GBs) have a significant influence on material properties. In the present paper, we calculate the energies of eleven low-Σ () symmetrical tilt GBs and two twist GBs in ferromagnetic bcc iron using first-principles density functional theory (DFT) calculations. The results demonstrate the importance of a sufficient sampling of initial rigid body translations in all three directions. We show that the relative GB energies can be explained by the miscoordination of atoms at the GB region. While the main features of the studied GB structures were captured by previous empirical interatomic potential calculations, it is shown that the absolute values of GB energies calculated were substantially underestimated. Based on DFT-calculated GB structures and energies, we construct a new d-band orthogonal tight-binding (TB) model for bcc iron. The TB model is validated by its predictive power on all the studied GBs. We apply the TB model to block boundaries in lath martensite and demonstrate that the experimentally observed GB character distribution can be explained from the viewpoint of interface energy.

A defect model for UO2+x based on electrical conductivity and deviation from stoichiometry measurements

Electrical conductivity of UO2+x shows a strong dependence upon oxygen partial pressure and temperature which may be interpreted in terms of prevailing point defects. A simulation of this property along with deviation from stoichiometry is carried out based on a model that takes into account the presence of impurities, oxygen interstitials, oxygen vacancies, holes, electrons and clusters of oxygen atoms. The equilibrium constants for each defect reaction are determined to reproduce the experimental data. An estimate of defect concentrations and their dependence upon oxygen partial pressure can then be determined. The simulations carried out for 8 different temperatures (973–1673 K) over a wide range of oxygen partial pressures are discussed and resulting defect equilibrium constants are plotted in an Arrhenius diagram. This provides an estimate of defect formation energies which may further be compared to other experimental data or ab-initio and empirical potential calculations.

Modeling of amorphous SiCxO6/5 by classical molecular dynamics and first principles calculations

Polymer-derived silicon oxycarbide (SiCO) presents excellent performance for high temperature and lithium-ion battery applications. Current experiments have provided some information on nano-structure of SiCO, while it is very challenging for experiments to take further insight into the molecular structure and its relationship with properties of materials. In this work, molecular dynamics (MD) based on empirical potential and first principle calculation were combined to investigate amorphous SiCxO6/5 ceramics. The amorphous structures of SiCO containing silicon-centered mix bond tetrahedrons and free carbon were successfully reproduced. The calculated radial distribution, angular distribution and Young’s modulus were validated by current experimental data, and more details on molecular structure were discussed. The change in the slope of Young’s modulus is related to the glass transition temperature of the material. The proposed modeling approach can be used to predict the properties of SiCO with different compositions.

A computational study of yttria-stabilized zirconia: I. Using crystal chemistry to search for the ground state on a glassy energy landscape

Yttria-stabilized zirconia (YSZ), a ZrO2-Y2O3 solid solution that contains a large population of oxygen vacancies, is widely used in energy and industrial applications. Past computational studies correctly predicted the anion diffusivity but not the cation diffusivity, which is important for material processing and stability. One of the challenges lies in identifying a plausible configuration akin to the ground state in a glassy landscape. This is unlikely to come from random sampling of even a very large sample space, but the odds are much improved by incorporating packing preferences revealed by a modest sized configurational library established from empirical potential calculations. Ab initio calculations corroborated these preferences, which prove remarkably robust extending to the fifth cation-oxygen shell about 8 Å away. Yet because of frustration there are still rampant violations of packing preferences and charge neutrality in the ground state, and the approach toward it bears a close analogy to glass relaxations. Fast relaxations proceed by fast oxygen movement around cations, while slow relaxations require slow cation diffusion. The latter is necessarily cooperative because of strong coupling imposed by the long-range packing preferences.

Theoretical Investigations for Strain Relaxation and Growth Mode of InAs Thin layers on GaAs(111)A

The growth mode of InAs/GaAs(111)A is systematically investigated using our macroscopic theory with the aid of empirical potential calculations that determine parameter values used in the macroscopic theory. Here, stacking-fault tetrahedron (SFT) found in InAs/GaAs(111)A and misfit dislocation (MD) formations are employed as strain relaxation mechanisms. The calculated results reveal that the MD formation occurs at the layer thickness h about 7 monolayers (MLs). Moreover, we found that the SFT forming at h about 4 MLs makes surface atoms move upward to reduce the strain energy to promote the two dimensional (2D) growth. Therefore, the SFT in addition to the MD plays an important role in strain relaxation in InAs thin layers on GaAs(111)A. The macroscopic free energy calculations for the growth mode imply that the InAs growth on the GaAs(111)A proceeds along the lower energy path from the 2D-coherent (h ≤ 4 MLs) to the 2D-MD (h ≥ 7 MLs) via the 2D-SFT (4 MLs ≤ h ≤ 7 MLs). Consequently, the 2D growth on the InAs/GaAs(111)A results from strain relaxation due to the formation of the SFT near the surface and the subsequent MD formation at the interface.

Size, Shape and Structure Dependent Cohesive Energy of Gold Nanoparticles: Empirical Many-Body Potential Energy Function Calculations

A thermodynamic model based on an empirical many-body potential energy function is used, to investigate the stability and energetic features of gold nanoparticles as a function of both size and shape. The concept of a localized atomic bond for surface atoms, and a non-relaxed atomic bond for the interior atoms is used. Among the five different investigated shapes, the qualitative behaviour for the cohesive energy per atom of gold nanoparticles with size range 200 1000 atoms, the general behaviour prefers; sphere < cubic <cuboctahedron < truncated octahedron < octahedron. On the other hand, the surface energy for gold nanoparticles is observed to be prominent in the nano-scale and it differs from shape to another, and falls down fast in the size range 200 < n < 1000 atoms having general trend; sphere < cubic <octahedron < truncated octahedron < cuboctahedron.

Multiscale simulation of xenon diffusion and grain boundary segregation in UO2

In light water reactor fuel, gaseous fission products segregate to grain boundaries, resulting in the nucleation and growth of large intergranular fission gas bubbles. The segregation rate is controlled by diffusion of fission gas atoms through the grains and interaction with the boundaries. Based on the mechanisms established from earlier density functional theory (DFT) and empirical potential calculations, diffusion models for xenon (Xe), uranium (U) vacancies and U interstitials in UO2 have been derived for both intrinsic (no irradiation) and irradiation conditions. Segregation of Xe to grain boundaries is described by combining the bulk diffusion model with a model for the interaction between Xe atoms and three different grain boundaries in UO2 (Σ5 tilt, Σ5 twist and a high angle random boundary), as derived from atomistic calculations. The present model does not attempt to capture nucleation or growth of fission gas bubbles at the grain boundaries. The point defect and Xe diffusion and segregation models are implemented in the MARMOT phase field code, which is used to calculate effective Xe and U diffusivities as well as to simulate Xe redistribution for a few simple microstructures.

Diffusion of Zr, Ru, Ce, Y, La, Sr and Ba fission products in UO2

The diffusivity of the solid fission products (FP) Zr (Zr4+), Ru (Ru4+, Ru3+), Ce (Ce4+), Y (Y3+), La (La3+), Sr (Sr2+) and Ba (Ba2+) by a vacancy mechanism has been calculated, using a combination of density functional theory (DFT) and empirical potential (EP) calculations. The activation energies for the solid fission products are compared to the activation energy for Xe fission gas atoms calculated previously. Apart from Ru, the solid fission products all exhibit higher activation energy than Xe. For all solid FPs except Y3+, the migration of the FP has lower barrier than the migration of a neighboring U atom, making the latter the rate limiting step for direct migration. An indirect mechanism, consisting of two successive migrations around the FP, is also investigated. The calculated diffusivities show that most solid fission products diffuse with rates similar to U self-diffusion. However, Ru, Ba and Sr exhibit faster diffusion than the other solid FPs, with Ru3+ and Ru4+ diffusing even faster than Xe for T<1200K. The diffusivities correlate with the observed fission product solubility in UO2, and the tendency to form metallic and oxide second phase inclusions.

Structures, Thermal Stability, and Chemical Activity of Crown-Jewel-Structured Pd–Pt Nanoalloys

In this work, the equilibrium structures, thermal stability, and chemical activity of crown-jewel (CJ)-structured Pd–Pt nanoalloys with highly symmetric cuboctaheral (Cubo), decahedral (Dec), and icosahedral (Ico) structures are studied by using molecular simulation based on the Gupta empirical potential and density functional theory (DFT) calculations. It is found that both the melting temperature and stability of CJ-structured Pd–Pt nanoalloys with the same size follow the order of Ico > Dec > Cubo, which agrees with the rule that the more stable the cluster, the higher the melting point. In addition, the melting temperature of CJ-structured Pd–Pt nanoalloys with the same morphology is of the same linear behavior with the inverse diameter of the clusters, which is consistent with the Pawlow’s law. The adsorption properties of O on these Pd–Pt nanoalloys are studied to model the chemical activity of these nanoalloys. For the CJ-structured Pd12Pt43, the adsorption strength of O follows the order of Dec < ...