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This work revises the concept of defects in crystalline solids and proposes a universal strategy for their characterization at the atomic scale using outlier detection based on statistical distances. The proposed strategy provides a generic measure that describes the distortion score of local atomic environments. This score facilitates automatic defect localization and enables a stratified description of defects, which allows to distinguish the zones with different levels of distortion within the structure. This work proposes applications for advanced materials modelling ranging from the surrogate concept for the energy per atom to the relevant information selection for evaluation of energy barriers from the mean force. Moreover, this concept can serve for design of robust interatomic machine learning potentials and high-throughput analysis of their databases. The proposed definition of defects opens up many perspectives for materials design and characterization, promoting thereby the development of novel techniques in materials science.

Empirical potentials using embedded atom method are developed for Fe, mainly to study irradiation-induced defects such as self-interstitial atom clusters or dislocation loops. The potentials are fitted using experimental values of solid-state properties, ab initio formation energies of basic point defects and ab initio forces acting on the atoms in the liquid or random state configurations. Various bulk and defect properties are compared to validate the transferability of the new potential. In this paper, we also investigate the energetic landscape of C15 self-interstitial atom clusters. In order to simplify and to facilitate the construction of lowest energy configurations in the complex energy landscape of C15 clusters, we test and propose three selection rules.

The properties of nanometric-sized helium bubbles in silicon have been investigated using both spatially resolved electron-energy-loss spectroscopy combined with a recently developed method, and molecular-dynamics simulations. The experiments allowed for an accurate determination of size, aspect ratio, and helium density for a large number of single bubbles, whose diameters ranged from 6 to 20 nm. Very high helium densities, from 60 to 180 He nm −3 , have been measured depending on the conditions, in stark contrast with previous investigations of helium bubbles in metal with similar sizes. To supplement experiments on a smaller scale, and to obtain insights into the silicon matrix state, atomistic calculations have been performed for helium bubbles in the diameter range 1-13 nm. Molecular-dynamics simulations revealed that the maximum attainable helium density is critically related to the strength of the silicon matrix, which tends to yield by amorphization at the highest density levels. Calculations give helium density values for isolated single bubbles that are typically lower than measurements. However, excellent agreement is recovered when the interactions between bubbles and the presence of helium interstitials in the matrix are taken into account. Both experiments and numerical simulations suggest that the Laplace-Young law cannot be used to predict helium density in nanometric-sized bubbles in a covalent material such as silicon.

The evolution of nanometric helium bubbles in silicon has been investigated using spatially resolved electron energy-loss spectroscopy during in situ annealing in the transmission electron microscope. This approach allows the simultaneous determination of both the morphology and the helium density in the bubbles at each step of the annealing. Structural modification and helium emission from bubbles of various diameters in the range 7.5 to 20 nm and various aspect ratios of 1.1 to 1.9 have been studied. We clearly show that helium emission takes place at temperatures where bubble migration had hardly started. At higher temperatures, the migration (and coalescence) of voids is clearly revealed. For helium density lower than 150 He nm −3 , the Cerofolini's model taking into account the thermodynamical properties of an ultradense fluid reproduces well the helium emission from the bubbles, leading to an activation energy of 1.8 eV. When bubbles exhibit a higher initial helium density, the Cerofolini's model fails to reproduce the helium emission kinetics. We ascribe this to the fact that helium may be in the solid phase and we propose a tentative model to take into account the properties of the solid.

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Combining classical molecular dynamics and first-principles DFT calculations, we perfom an extensive investigation of low energy configurations for He n V m complexes in silicon. The optimal helium fillings are hence determined for V 1 , V 2 , and V 6 (figure on the right), and the structures formed by helium atoms arrangements in the vacancy defect are analyzed. For V 1 and V 2 , the He atoms structure is mainly controled by the host silicon matrix, whereas a high density helium packing is obtained for V 6 . For the latter, we estimate a helium density of about 170 He nm −3 in the center of the hexa-vacancy at the optimal helium filling.Relaxed structures obtained from DFT calculations for configurations with the lowest formation energies: (a) He 14 V 1 , (b) He 20 V 2 , and (c) He 40 V 6 .

The formation and growth of helium-filled cavities in silicon have been investigated using both molecular dynamics simulations and rate equation cluster dynamics calculations. This multiscale approach allowed us to identify atomic scale mechanisms involved in nucleation and early growth steps, and to follow their dynamics over experimental timescales. We especially focus our analyses on the influence of helium. Our results first suggest that both Ostwald ripening and migration-coalescence mechanisms are jointly activated during bubble growth. We also discover that an original mechanism, based on the splitting of bubbles, could have a significant contribution. Overall, helium atoms are found to delay growth, proportionally to their concentration. This can be clearly observed at the nanosecond timescale. However, for longer timescales, cluster dynamics calculations also reveal periods of accelerated growth for specific helium concentrations. Finally, it is determined that the main effect of Si interstitials is to impede bubble growth, due to an early recombination with vacancies.