Classical many-body potentials that have a rather low number of parameters and are based on physically-inspired functional forms are expected to display a reasonable transferability, though their flexibility is limited. Ways to improve them when their transferability is unsatisfactory and when new interesting structures/targeted properties emerge are not clearly established. Here, model screening and sensitivity analysis techniques are combined to get insights on the intrinsic capabilities of classical potentials for typical sets of targeted properties, and we propose to use the outcomes of the sensitivity analysis to improve potential performances. Usefulness is illustrated on existing simple second moment potentials (SMA), and we refit one such model for the study of small irradiation defects in α-zirconium. Application of this approach is recommended for more complex interaction models with up to tens of parameters, such as other classical many-body potentials or tight-binding electronic structure models, that are possible to re-optimize while gaining a better understanding of the role of their parameters.

Grain boundaries (GB) play a major role in the mechanical properties of steel. We model here the most common Σ5 (210) and (310) [001] tilt boundaries in fcc Ni and fcc Fe which are main components of steel, paying particular attention to the effect of magnetism on the relative stabilities of the competing GB structures and on their main characteristics (excess volume, microstructure, stress profile, ...). To this aim we develop a new interatomic potential in the second moment approximation of the Tight-Binding scheme which accounts for magnetism in an effective manner in order to relax the structures at T=0K within quenched molecular dynamics simulations. Similar results are obtained for Fe and Ni, even though magnetic effects are found much more important in the former case. More precisely, magnetism significantly lowers the excess GB energy and volume, strongly reduces the stress in the GB region, and can even modify the microstructure in some cases. Finally a local analysis is proposed which allows to classify the relative stabilities of the different structures in terms of only a few given atomic sites.

We present a detailed methodology for treating local concentration dependency of pairwise interactions in Ising-based Monte-Carlo. The procedure is described through the example of interstitial ordering processes in zirconium hydrides, studied by canonical Monte-Carlo based on a concentration-dependent Tight-Binding Ising Model. The path leads to build a phase diagram of hydrogen-vacancy ordering on interstitial tetrahedral sublattice of face-centered cubic Zr-H.

We use a Tight-Binding Ising Model (TBIM) with effective pair interactions depending on concentration able to reproduce the bulk phase diagram of Pt-Ag system and in particular its strong asymmetry as a function of composition. This system presents both an L11 ordered structure around equiconcentration and a wide miscibility gap on the Pt-rich side. Using Monte Carlo simulations in canonical and semi-grand canonical ensembles, we succeed in reproducing qualitatively the bulk phase diagram and we study the equilibrium configurations of the (111) and (100) surfaces, and truncated octahedron clusters. The infinite surfaces reproduce sensibly the bulk phase diagram except that they present a strong silver surface segregation limited to the top surface layer. The clusters behave differently due to the reduction of the miscibility gap when decreasing the cluster size and present two typical ordered phases related to the L11 phase: one characterized by pure concentric atomic layers starting from silver surface layer which extends up to the center of the cluster, and the other one at higher silver concentration where the L11 phase is perfectly oriented along one (111) direction of the cluster. The later one has already been reported by recent experiments [1].

We present here a theoretical study of ordering processes in metal-hydrogen compounds based on a generalized perturbation method and on tight-binding coherent potential approximation. This approach is illustrated for zirconium hydrides, in which case we demonstrate that a cluster expansion of the ordering energy can be limited to effective pair interactions, the leading one being between hydrogen atoms in third-neighbor positions. These results are quantitatively confirmed by comparison to density functional theory calculations and qualitatively interpreted through orbital symmetry analysis. The method is then applied first to draw a preliminary Zr-H phase diagram and then to characterize the effect of lattice deformation on the ordering processes in zirconium hydrides.

During in-pile operation, fission gases such as xenon are produced and precipitates in nuclear fuel as nano-bubbles, modifying the thermomechanical properties of UO2 fuel matrix. This work aims at precising the nature and the scope of the xenon-UO2 matrix interaction. To this end, we present a theoretical study of xenon adsorption on UO2 surfaces, based on Monte Carlo simulation and semi-empirical interatomic potential. We determine xenon adsorption isotherms on previously relaxed (111) and (100) UO2 surfaces, and we compare them to the incorporation ones inside an empty box in order to isolate interface effects. A specific attention is given to the microstructure of xenon in these systems. Finally, an analysis of the mechanical properties (pressure and stress profiles near by the surface) is achieved in order to investigate the sensitivity to surface orientation and to get pertinent quantities that will supply micromechanical models at higher scale.

Surface segregation and bulk ordering are characterized by Monte Carlo simulations using a Tight-Binding interatomic potential within the Second Moment Approximation of the density of states (TB-SMA potential) which reproduces the bulk ordered phases and the surface segregation reversal between the dense (111) and (100) surfaces (Pt segregation) and the more opened (110) one (Co segregation). Among the different surface superstructures, most are simply the surface termination of bulk ordered phases and have been observed experimentally, but we predict also purely bidimensional phases such as the (root 3 x root 3)R30(111) or the c(2 x 2)(110) ones with no equivalence in the bulk alloy. Such (root 3 x root 3)R30(111) phase should be stable on nanoalloys where the (111) facets are prevailing. Finally, we show that the c(2 x 2) superstructure of the CoPt(100) surface, which represents the mixed variant of the L1(0) phase, remains stable above the bulk order/disorder critical temperature.

Ordering alloys are interesting systems due to the link between atom arrangement and properties, for instance in the field of catalysis for Pt-based alloys. In fact, Co-Pt system presents higher activity in the electrochemical oxygen reduction reaction than pure Pt [1], whereas Pt-Ag presents skills for plasmonic catalysis [2,3]. According to their bulk phase diagram, these systems are quite different; Co-Pt has three ordered phases (L12 at Co3Pt and CoPt3, and L10 at CoPt) while Pt-Ag holds one (L11 at PtAg) which is different from the one observed in Co-Pt. Moreover Silver presents a strong surface segregation tendency in Pt-Ag system whereas neither Co, nor Pt does in Co-Pt. The question arises if bulk ordering remains in nano-sized particles and how surface segregation accommodates chemical ordering. We propose here a theoretical study of surface segregation and chemical ordering.

Modelling the segregation of the various chemical species in the vicinity of crystallographic defects in FeNi alloys is essential because it affects the macroscopic properties of these materials, which are widely used in technological applications. We present here a theoretical study of surface segregation, within a mean-field approach based on the tight-binding Ising model grounded on density functional theory calculations. The most important result is that, although FeNi presents none of the driving forces (i.e. surface energy, size mismatch) which generally favour surface enrichment in the same element in the whole range of concentrations, there exists a wide temperature range in which Ni is found to segregate at the surface irrespective of the concentration. This is due to a complex interplay between magnetic and ordering/phase separation effects.

Most of tight-binding studies of transition metal based systems deviating from perfect bulk (surfaces, nanoparticles, alloys) are based on local charge neutrality rules per site, per valence orbital and per element. Unfortunately, such rules do not hold per spin when interested in magnetic elements. We present here a simple way to characterize the variation of the magnetic moment with the environment and to generalize the tight-binding expression of the energy to account for magnetism. This is illustrated in the particular case of cobalt, going from perfect pure bulk to surface, nanoparticles and then CoPt alloy.

We propose here a general methodology to derive tight-binding potentials accounting for spd hybridization in transition metals, dealing simultaneously with electronic structure and energy properties. This methodology is illustrated for zirconium which is largely used for technological applications, in particular in the nuclear industry, and whose modelling is known to be complex and challenging. Such potentials are very promising. Their fits have a clear physical meaning with a limited amount of parameters and their complexity can be adjusted as a function of the problem under consideration.

An accurate description of the local electronic structure is necessary for guiding the design of materials with targeted properties in a controlled way. For complex materials like nanoalloys, self-consistent tight-binding calculations should be a good alternative to ab initio methods, for handling the most complex and large systems (hundreds to thousands of atoms), provided that these parameterized method is well founded from ab initio ones that they intend to replace. Ab initio calculations (density functional theory) enabled us to derive rules for charge distribution as a function of structural change and alloying effects in Co and Pt based systems, from bulk to nanoalloys. A general local neutrality rule per site, orbital and species was found. Based on it, self-consistent tight-binding calculations could be implemented and applied to CoPt nanoalloys. A very good agreement is obtained between tight-binding and DFT calculations in terms of local electronic structure.

Oxidation of a dilute Si(Ge) alloy is modeled using an original protocol based on molecular dynamicssimulation and rules for the oxygen insertions. These rules, deduced from ab-initio calculations,favor the formation of SiO2 against GeO2 oxide which leads to segregation of Ge atoms into the alloyduring the oxidation front advance. Ge condensation is then observed close to the SiO2/Ge interfacedue to the strain induced by oxidation in this region. From the analysis of the simulations process, wepropose a one-dimensional description of Ge condensation which reproduces the evolution of the Geconcentration during oxidation of the SiGe alloy.

The influence of stress on the distribution of slow-diffusing substitutional impurities in the vicinity of a dislocation loop in Si and Ge bulk was theoretically investigated, at the atomic scale, using the Si and Ge Stillinger–Weber potentials via Monte Carlo and kinetic Monte Carlo simulations. The dislocation loop was modeled by an extra atomic plane introduced between two (111) planes. The calculations were performed at high temperature, for which impurity diffusion was enabled. The influence of atomic size effect on Cottrell atmosphere formation was investigated considering the difference of atomic volume between Si and Ge. The dislocation loop elastic field was found to prevent the accumulation of substitutional atoms in the vicinity of the dislocation. However, the calculations suggest that substitutional impurities can occupy interstitial sites close to the dislocation loop. In this case, the elastic field surrounding the dislocation loop can promote Cottrell atmosphere formation mainly if the impurity exhibits a larger atomic radius than the matrix atoms (Si or Ge).

Modeling metal hydrides is motivated by both the fundamental questions that are revealed and by their diverse applications in the domain of materials for energy. In particular, zirconium alloys are used in water-cooled nuclear reactors and can be embrittled by hydride precipitation. This motivates the development of theoretical tools to model, understand and predict the behavior of zirconium hydrides. In order to explore the Zr-H phase diagram and clarify dissolution/precipitation mechanisms and kinetics, one requires thermodynamic simulations. They have to be based on an energetic model that allows differentiating the various Zr-H phases. We present and validate here a new Zr-H interatomic potential developed in the framework of Tight-Binding approximation, accounting for spd hybridization, which satisfies this requirement.

Modelling oxide surface behaviour is of both technological and fundamental interest. In particular, in the case of the UO2 system, which is of major importance in the nuclear industry, it is essential to account for the link between microstructure and macroscopic mechanical properties. Indeed micromechanical models at the mesoscale need to be supplied by the energetic and stress data calculated at the nanoscale. In this framework, we present a theoretical study, coupling an analytical model and thermostatistical simulation to investigate the modifications induced by the presence of a surface regarding atomic relaxation and energetic and stress profiles. In particular, we show that the surface effective thickness as well as the stress profile, which are required by micromechanical approaches, are strongly anisotropic.

Monte Carlo simulations within a tight-binding Ising model (TBIM) have been performed on bulk, surfaces, and nanoclusters of Co1-c Pt-c alloys in order to describe and understand the competition or synergy between surface segregation and chemical ordering phenomena in nanoalloys. Considering effective pair interactions (EPIs) up to the third neighbors, we put in evidence new ordered phases at low temperature in the Co-Pt bulk phase diagram. On the infinite (100) and (111) surfaces, the Pt surface segregation leads to select the Pt-rich plan at the surface without modification of the bulk ordering in the (100) orientation but with an extension of the ordering on a larger composition range in the (111) orientation as compared to the bulk. The truncated octahedron clusters of 405 and 1289 atoms are studied. Their chemical structure is compared in their core with the bulk phase diagram and in their facets with the (111) and (100) infinite surfaces segregation isotherms. The cluster core presents an asymmetry as compared to the bulk phase diagram. The (111) facets are similar to the (111) surface, whereas the (100) facets present geometrical frustrations for the segregation versus core ordering.

We present a theoretical study of xenon incorporation in UO2 nanocavities, by means of Grand Canonical Monte Carlo calculations based on semi-empirical potentials. We first characterize the reconstruction of the matrix around an empty cavity which leads to a stoechiometry change from UO2 to UO in this region. Then, we determine xenon adsorption isotherms which exhibit an abrupt transition from a dilute phase to a dense one and an increase in the density of the latter phase as a function of temperature. This last result is attributed to a vibrational entropy effect by means of a mean field analysis. Finally, the pressure calculation inside the bubble proves the limitations of the usual mesoscopic models based on gas state behaviour.

We present here an analytical method, based on the kinetic theory, to determine the impact of defects such as cavities on the thermal conductivity of a solid. This approach, which explicitly takes into account the effects of internal pore surfaces, will be referred to as the Phonon Interface THermal cONductivity (PITHON) model. Once exposed in the general case, this method is then illustrated in the case of uranium dioxide. It appears that taking properly into account these interface effects significantly modifies the temperature and porosity dependence of thermal conductivity with respect to that issued from either micromechanical models or more recent approaches, in particular, for small cavity sizes. More precisely, it is found that if the mean free path appears to have a major effect in this system in the temperature and porosity distribution range of interest, the variation of the specific heat at the surface of the cavity is predicted to be essential at very low temperature and small sizes for sufficiently large porosity.

Accurate prediction of local properties of transition-metal nanoalloys from the electronic structure is a challenge for building new materials with novel properties in a controlled way. To this aim, developing unified descriptions of local electronic states as a function of a minimal set of parameters is the way to disentangle structural and chemical effects. This is achieved here within sp-d tight-binding calculations using a self-consistent procedure taking into account both the changes in the structural environment (coordination effect) and in the chemical one (alloying effect). From these calculations, trends in the distributions of energy electronic states are obtained through band shifts and widths allowing one to study in a systematic way ordering tendency and local properties in nanoalloys in a wide range of sizes and structural complexity.