The manipulation of individual colloidal particles using optical tweezers has allowed vacancies to be created in two-dimensional (2d) colloidal crystals, with unprecedented possibility of real-time monitoring the dynamics of such defects (Nature 413, 147 (2001)). In this Letter, we employ molecular dynamics (MD) simulations to calculate the formation energy of single defects and the binding energy between pairs of defects in a 2d colloidal crystal. In the light of our results, experimental observations of vacancies could be explained and then compared to simulation results for the interstitial defects. We see a remarkable similarity between our results for a 2d colloidal crystal and the 2d Wigner crystal (Phys. Rev. Lett. 86, 492 (2001)). The results show that the formation energy to create a single interstitial is 12% − 28% lower than that of the vacancy. Because the pair binding energies of the defects are strongly attractive for short distances, the ground state should correspond to bound pairs with the interstitial bound pairs being the most probable.
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Using path‐integral Monte Carlo (PIMC) simulations in the isothermal‐isobaric ensemble, the authors investigate the quantum effects on structural and thermodynamic properties of cubic boron nitride (c‐BN) at pressures varying from 0 up to 162 GPa. The Albe–Tersoff potential is employed to describe the interatomic interactions. The differences between the obtained lattice parameters of the crystal from the PIMC simulation and their experimental values are less than 0.1%. The quantum effects of the atomic zero‐point motion are significant for temperature lower than 1000 K. At ambient pressure, the quantum contribution to the lattice vibrational energy overcomes the classical one for T≲386 K. This temperature dependent crossover shifts to higher temperature with increasing pressure. The ratio of the kinetic to potential vibrational energy is used to quantify the anharmonicity of the system, indicating that the c‐BN is essentially an anharmonic crystal. Furthermore, the root‐mean‐square displacement (RMSD) analysis shows that the quantum effects become insignificant for temperature higher than 1000 K where different atoms (boron and nitrogen) behave similarly. However, for lower temperatures, the quantum effects arise and the atoms have quite different behaviors resembling those from rare‐gas solids (boron) and covalent materials (nitrogen).
Mixed Uranium-Plutonium Carbides are candidate fuels for 4th generation nuclear reactors. In this frame, a research program is underway at CEA (French alternative and atomic energies commission) to gather basic properties of such material to get a better understanding of in-pile behaviour. In the field of solid state physics, the measurement of transport properties and in particular diffusion coefficient requires the preparation of uranium carbide samples with very specific characteristics: very low oxygen content, mono-phased UC, high density (greater than 95% of the theoretical one), coarse microstructure, accurate samples geometry. In this paper the Process development is presented to manufacture such samples and the specific equipments which have been set up in glove-box. Characterizations of the first samples are also given.
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