This computational study delves into the intricate interplay of alloying elements on the generation, recombination, and evolution of irradiation-induced defects. Molecular dynamics simulations were conducted for collision cascades at room temperature, spanning a range of primary knock-on atom energies from 1 to 10 keV. The investigation encompasses a series of model crystals, progressing from pure Ni to binary concentrated solid solution alloys (CSAs) such as NiFe20, NiFe, NiCr20, and NiFeCr20 CSA. We observe that materials rich in Cr actively facilitate dislocation emissions and induce the nucleation of stacking fault tetrahedra in the proximity of nanovoids, due to Shockley partial interactions. This result is validated by molecular static simulations, which calculate the surface, vacancy, and defect formation energies. Among the various shapes considered, the spherical void proves to be the most stable, followed by the truncated octahedron and octahedron shapes. On the other hand, the tetrahedron cubic shape is identified as the most unstable, and stacking fault tetrahedra exhibit the highest formation energy. Notably, among the materials studied, NiCr20 and NiFeCr20 CSAs stood out as the sole alloys capable of manifesting this mechanism, mainly observed at high impact energies.

This study employs molecular dynamics simulations to investigate the effect of varying water content (0–25 wt.%) and Si/Al ratio (1–3) on the atomistic structure and elastic properties of sodium aluminosilicate hydrates (N–A–S–H) gels. Results show that water molecules break the aluminosilicate framework by forming hydroxyl groups on AlO 4 and SiO 4 units. It induces the release of sodium from its charge‐balancing role, which in turn solubilizes in water. The framework unfolds while the pores are progressively filled by water, and it leads to depolymerization. The elastic properties are therefore largely affected. In one hand, the framework of the gels and the filling of the pores by water control the evolution of the bulk modulus. On the other hand, subtle effects included in the density, where porosity and its saturation, density of the dry framework, and Si/Al ratio appear together, drive the Young's modulus.

This study presents the development of a synthesis route for sodium aluminosilicate hydrate (N-A-S-H) gels with various Si/Al ratios carried out at ambient temperature. The synthesis is based on a simple "sol-gel" method, where commercial reactants are used to provide highly reactive Si an Al sources. Comprehensive characterization, including scanning electron microscope-energy-dispersive spectroscopy (SEM-EDS), gas pycnometry, inductively coupled plasma (ICP), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and magic angle spinning (MAS) nuclear magnetic resonance (NMR) (27Al, 23Na, 29Si, and 1H), is employed to verify the morphology, density, chemical composition, long-range order, and local structure of the gels. Our results show that the synthesized gels are pure, amorphous, and homogeneous with Si/Al final ratio ranging from 1.22 to 2.23. Structural analysis of the gels indicates the synthesis of compounds with high degree of geopolymerization, which are representative of N-A-S-H gel formed in sodium-based geopolymers. center dot A new route to synthesize pure sodium aluminosilicate hydrate (N-A-S-H) gels representative of N-A-S-H formed in sodium-based geopolymers is presented.center dot N-A-S-H gels are synthesized at room temperature with various Si/Al ratios.center dot Synthesized gels are amorphous, homogeneous, and well-geopolymerized. image

The elastic moduli and mechanical properties at the onset of crack in nanocrystalline and nanoporous (Ni, Fe)Cr2O4 compounds with a spinel structure are investigated by molecular dynamics simulations. The polycrystalline structures generated contain nanograins from 2.5 to 30 nm in diameter. These structures are representative of the internal corrosion layer in nickel-based alloys. These simulations enabled us to establish the evolution of elastic moduli as a function of the composition, porosity, and grain size of the polycrystals. From this evolution, the initial database for the elastic properties of corrosion layers based on von Bertalanffy growth functions was determined. The onset of crack in polycrystals is also investigated via uniaxial tensile and shear deformation. Under shear deformation, flow stress as a function of grain size follows normal and inverse Hall-Petch regimes. The regime change occurs for grain sizes around 10 nm. For grain sizes under this threshold, shear banding involving collective translation and rotation of nanograins dominates the plastic deformation. For grain sizes greater than 10 nm, phase transition inside grains from a spinel to a post-spinel-like structure is observed as well. In that case, phase transition dominates the plastic deformation. Under uniaxial tensile deformation, intergranular decohesion occurs. The general law as a function of grain size for toughness, which is the material's capacity to absorb elastic and plastic energy prior to failure, is also established.

The response of MgO periclase to irradiation is investigated by means of molecular dynamics simulations, mimicking irradiation by Frenkel pairs accumulation. Both the calculated lattice and volume swellings, which refer, respectively, to the lattice and total volume changes reproduce well the experimental measures. The two diverge at around 0.2 dpa, above which lattice and volume swellings follow separate trends. Below this value, dislocation loops nucleate from point defects clusters, built up by progressive aggregation of both magnesium and oxygen interstitials. Very small 12⟨110⟩ loops lying in {001} planes and made of (MgO)6 interstitials could be characterized. They serve as seeds for the subsequent growth of dislocation loops in all three {110}, {001}, and {111} planes, which then follows a sublinear law. The 12⟨110⟩ loops lying in the {011} planes become dominant as loop diameters increase beyond 15 nm. Above 0.2 dpa, we observe (i) the relative decrease of lattice swelling mainly because the very dense dislocations loops recombine and stabilize into less dense dislocation forests and, concomitantly, (ii) the fast increase of volume swelling caused by the occurrence of significant voids of up to 32 vacancies.

It is generally considered that the elementary building blocks of defects in face-centred cubic (fcc) metals, e.g., interstitial dumbbells, coalesce directly into ever larger 2D dislocation loops, implying a continuous coarsening process. Here, we reveal that, prior to the formation of dislocation loops, interstitial atoms in fcc metals cluster into compact 3D inclusions of A15 Frank-Kasper phase. After reaching the critical size, A15 nano-phase inclusions act as a source of prismatic or faulted dislocation loops, dependent on the energy landscape of the host material. Using cutting-edge atomistic simulations we demonstrate this scenario in Al, Cu, and Ni. Our results explain the enigmatic 3D cluster structures observed in experiments combining diffuse X-ray scattering and resistivity recovery. Formation of compact nano-phase inclusions in fcc structure, along with previous observations in bcc structure, suggests that the fundamental mechanisms of interstitial defect formation are more complex than historically assumed and require a general revision. Interstitial-mediated formation of compact 3D precipitates can be a generic phenomenon, which should be further explored in systems with different crystallographic lattices.

Tuning magnetic and electronic transport properties in spinel oxides requires a faithful description between chemical composition and cation site-occupation. Here this challenge is addressed using Fe3-xCrxO4 thin films grown by oxygen-assisted molecular-beam epitaxy within a wide range of composition (0.0 <= x <= 1.2). Spec-troscopic measurements (e.g., X-ray magnetic circular dichroism), refined by theoretical simulations (e.g., crystal field multiplet), are performed to establish a quantitative link between chromium content, Fe2+/Fe3+ site-occupation and macroscopic physical properties of the layers. It is found that Fe3-xCrxO4 thin films (i) delay the transition from inverse to normal spinel configuration with increasing chromium content and (ii) promote collinear spin structure, at odds with bulk material. As a result, strong antiferromagnetic interactions are pre-served between spins in tetrahedral and octahedral spinel sublattices, so that chromium-rich thin films exhibit Curie temperatures above room temperature and higher magnetization. Electron hopping is also favored by this singular cation distribution and electronic band gap is smaller than expected for these thin films. The cation site-occupation is therefore a key feature to consider for applications of Fe3-xCrxO4 thin films in spintronics and photocatalysis, as it enables manipulation of magnetic properties (Curie temperature and magnetization) and band gap engineering.

To explain irradiation creep, several mechanisms have been proposed. Some are based on the effect of stress on either nucleation or growth of dislocation loops. To investigate these mechanisms in aluminum we combine in-situ transmission electron microscope irradiation under stress and two simulation approaches (object kinetic Monte-Carlo, molecular dynamics Frenkel pair accumulation). We observe the selectivity of Frank loop variants under electron irradiation and applied stress. When the stress is turned on after loop formation, there is no loop variant selectivity, suggesting the absence of preferential absorption on already formed loops. Object kinetic Monte-Carlo simulations, including the effect of stress on the diffusion of point defects, show no growth rate difference between loop variants. Frenkel pair accumulation simulations exhibit variant selectivity of nucleated loops. This shows that loop selectivity is due to preferential nucleation of well oriented loops under stress and not to differential growth of loops.

The relaxation volume (Orel), here determined per extra-atom or vacant site, of common crystalline defects in bcc iron (Fe) was calculated from molecular dynamics (MD) simulation cells containing defects of varying size and/ or density. To this end, we used both real and reciprocal space data: for the former, the change in the MD cell volume was calculated, while for the latter, we computed X-ray diffraction reciprocal space maps to evaluate the change in the lattice parameter. We show that (110) dumbbell self-interstitial atoms have the largest Orel,-1.5 atomic volume (-1.5 O0). C15 clusters of size 12 and 48 atoms show Orel of -0.91 O0 and -0.98 O0, respectively, and similar values are found for1/2(111) and (100) interstitial dislocation loops, with Orel -0.905 O0 and Orel -0.873 O0, respectively. Single vacancies are characterized by a negative Orel,--0.11 O0. For cavities, Orel rapidly increases to approach zero as the clusters grow. Using these values, we managed to predict (with an accuracy better than 2 %) the lattice strain in MD cells containing several types of defects, which in-dicates that the relaxation volumes can be summed up to estimate the microscopic (i.e., lattice) volume change.

Atomic-scale simulations, and in particular molecular dynamics (MD), are key assets to model the behavior of the structure of materials under the action of external stimuli, say temperature, strain or stress, irradiation, etc . Despite the widespread use of MD in condensed matter science, some basic material characteristics remain difficult to determine. This is, for instance, the case for the long-range strain tensor, and its root-mean-squared fluctuations, in disordered materials. In this work, computational diffraction is introduced as a fast and reliable structural characterization tool of atomic-scale simulation cells in the case of irradiated single crystals. In contrast to direct-space methods, computational diffraction operates in the reciprocal space and is therefore highly sensitive to long-range spatial correlations. With the example of irradiated UO 2 single crystals, it is demonstrated that the normal strains, shear strains and rotations, as well as their root-mean-squared fluctuations (microstrain) and the atomic disorder, are straightforwardly and unambiguously determined. The methodology presented here has been developed with efficiency in mind, in order to be able to provide simple and reliable characterizations either operating in real time, in parallel with other analysis tools, or operating on very large data sets.

Molecular dynamics simulations of microstructural evolution under irradiation of (U,Pu)O-2 solid solutions have been carried out with the Frenkel pair accumulation method with two empirical potentials. Simulated X-Ray diffraction patterns have been systematically generated along the irradiation pathway from the computed atomistic configurations. Description of the X-Ray diffraction patterns for each of these well characterized damage microstructure snapshots is discussed. Attempt to correlate the features of the X-Ray diffraction peaks with the microstructural evolution with irradiation dose is also provided. Special attention is dedicated to the swelling effect. (C) 2022 Elsevier B.V. All rights reserved.

The influence of epitaxial strain on the electronic structure of α-Cr2O3(0001) thin films is probed by combining X-ray photoemission spectroscopy and crystal field multiplet calculations. In-plane lattice strain introduces distortions in the CrO6 octahedron and splits the 3d orbital triplet t2g into a1 + e orbitals. For relaxed thin films, the lines-shape of the Cr 2p core levels are well reproduced when the t2g subset is fully degenerated. In-plane tensile strain stabilizes a1 with respect to e orbitals, whereas compressive strain destabilizes a1 orbitals. Understanding these crystal field variations is essential for tuning the physical properties of α-Cr2O3 thin films.

a. Université Paris Saclay, CEA Saclay, Service de Corrosion et du Comportement des Matériaux dans leur Environnement, 91191 Gif‐sur‐Yvette, France b. Service de Physique de l’Etat Condensé (SPEC), CEA, CNRS UMR 3680, Université Paris Saclay, CEA Saclay c. CEMES-CNRS, Université de Toulouse, CNRS, 29 rue Jeanne Marvig, 31055 Toulouse, France d. Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin, BP 48, 91192 Gif-sur-Yvette, Cedex, France

Atomic scale simulations are a key element of modern science in that they allow to understand, and even predict, complex physical or chemical phenomena on the basis of the fundamental laws of nature. Among the different existing atomic scale simulation approaches, molecular dynamics (MD) has imposed itself as the method of choice to model the behavior of the structure of materials under the action of external stimuli, say temperature, strain or stress, irradiation, etc. Despite the widespread use of MD in condensed matter science, some basic material characteristics remain difficult to determine. This is for instance the case of the long-range strain tensor in heavily disordered materials, or the quantification of rotated crystalline domains lacking clearly defined boundaries. In this work, we introduce computational diffraction as a fast and reliable structural characterization tool of atomic scale simulation cells. As compared to usual direct-space methods, computational diffraction operates in the reciprocal-space and is therefore highly sensitive to longrange spatial correlations. With the example of defective UO2, it is demonstrated that the homogeneous strain tensor, the heterogeneous strain tensor, the disorder, as well as rotated crystallites are straightforwardly and unambiguously determined. Computational diffraction can be applied to any type of atomic scale simulation and can be performed in real time, in parallel with other analysis tools. In experimental workflows, diffraction and microscopy are almost systematically used together in order to benefit from their complementarity. Computational diffraction, used together with computational microscopy, can potentially play a major role in the future of atomic scale simulations.

Some AX2 binary compounds with the fluorite structure (space group Fm3̄m) are well-known examples of materials exhibiting transitions to ionic superconducting phases at high temperatures below their melting points. Such superionic states have been described as either highly defective crystals or part-crystal, part-liquid states where the A ions retain their crystalline order whilst the X ions undergo partial melting. However, no detailed description of the structure of these phases exists. We present here the results of our investigation of the structural changes that occur during these transitions and the structural characteristics of the resulting superionic materials. This work is based on atomic-scale molecular dynamics modelling methods as well as computational diffraction techniques. We employed a set of empirical potentials representing several compounds with the fluorite structure to investigate any potential-dependent effect. We show the importance of small-scale structure changes, with some local environments showing a hexagonal symmetry similar to what is seen in the scrutinyite structure that has been documented for example in UO2.

Ion beams delivered by particle accelerators are routinely used to emulate harsh, radiative environments and they also constitute the foundations of the modern microelectronics industry. To characterize irradiated materials, numerous experimental and computational techniques can be implemented, but it is extremely difficult to effectively intertwine them, and to compare the associated data. In the present work, we present an integrated, experimental and computational approach that uses a same set of molecular dynamics simulations to generate signals of Rutherford backscattering spectrometry in channelling condition and X-ray diffraction, with UO2 as a test-case material. From these signals, parameters to monitor the damage level are computed, compared and confronted with experimental data. Although the evolution of the strain and disordering kinetics obtained by simulations differ on an absolute scale from those obtained experimentally (a discrepancy inherent to the method used to generate the atomic-scale data), a very good relative agreement is obtained, which demonstrates the validity of the approach, hence providing a new tool for the fine study of irradiation effects in materials.

Ion beams delivered by particle accelerators are routinely used to emulate harsh, radiative environments and they also constitute the foundations of the modern microelectronics industry. To characterize irradiated materials, numerous experimental and computational techniques can be implemented, but it is extremely diffcult to effectively intertwine them, and to compare the associated data. In the present work, we present an integrated, experimental and computational approach that uses a same set of molecular dynamics simulations to generate signals of Rutherford backscattering spectrometry in channelling condition andX-ray diffraction, with UO2 as a test-case material. From these signals, parameters to monitor the damage level are computed, compared and confronted with experimental data. The good agreement that comes out demonstrates the validity of the approach, hence providing a new tool for the fine study of irradiation effects in materials.

The modelling of the thermo-kinetic properties of uranium-plutonium mixed oxide (MOX) is of utmost importance for optimizing its synthesis and for predicting its behaviour in Fast Breeder Reactors. Despite the stakes and likely because of experimental issues, little or no experimental data are available for the entire MOX system. We circumvent here the difficulties by developing a mobility database for plutonium using the DICTRA code. A well-established model of MOX formalized within the Compound Energy Formalism ensures the thermodynamic description. Rationalisation of the mobility parameters combined with the use of both cB Omega model and the few experimental data lead to a full and comprehensive description of plutonium self-diffusion in MOX for any plutonium content, O/M ratio and temperature. Additionally, we show that the observed plateau of the self-diffusion as a function of the oxygen to metal ratio (O/M) is related to the constant Pu3+ fraction for very low O/M ratio. Moreover, the observed minimum close to O/M = 2 is found for the lowest mobility of Pu4+.