Clusters of Oxygen Interstitials in UO2+x and α-U4O9: Structure and Arrangements.

We use the Second Moment Tight Binding with charge eQuilibration (SMTB-Q) advanced empirical potential to study clusters of oxygen interstitials in hyperstoichiometric UO2. This potential reproduces the trends obtained from density functional theory calculations. We exhibit a new form of cuboctahedron that proves to be the most stable oxygen cluster in UO2+x. Considering different types of random arrangements of these clusters, we reproduce the fact that UO2 and U4O9 are the only two stable phases at low temperature in their composition range. Focusing on U4O9 at low temperature, we obtain a model for the α-U4O9 phase, which exhibits the experimental R3c space group. We present the results of the original neutron diffraction experiments measured on U4O9 at 1.5 K and perform a Rietveld refinement of the calculated U4O9 structure, which enables us to propose a complete description of the α-U4O9 phase.

Effect of the variation of the electronic density of states of zirconium and tungsten on their respective thermal conductivity evolution with temperature.

The thermal conductivity of zirconium and tungsten above 500 K is calculated with atomistic simulations using a combination of empirical potentials molecular dynamics and density functional theory calculations. The thermal conductivity is calculated in the framework of Kubo-Greenwood theory. The obtained values are in quantitative agreement with experiments. The fact that the conductivity of Zr increases with temperature while that of tungsten is essentially constant is reproduced by the calculations. The evolution with temperature of the electronic density of states of these two pseudo-gap metals proves to explain the observed variations of the conductivity.

Origins of the high temperature increase of the thermal conductivity of transition metal carbides from atomistic simulations.

To understand the unexpected increase of the thermal conductivity of transition metal carbides at high temperatures, we calculate, with atomistic simulations, the thermal conductivity of zirconium carbide (ZrC). To account for the common substoichiometry of this material, various numbers of carbon vacancies are considered. The vibrational part of the conductivity is calculated by empirical potential molecular dynamics while the electronic part is calculated from density functional theory electronic structure with the Kubo-Greenwood formula on selected atomic configurations generated by the same empirical potential. We find that the vibrational part of the conductivity is negligible at temperatures higher than 1500 K. The increase of thermal conductivity with temperature is quantitatively reproduced in the calculations. It appears for all compositions and proves to rely entirely on its electronic component. Three phenomena are found responsible for the rise of the thermal conductivity with temperature: the semi-metallic shape of the electronic density of states, the additional electrical resistivity induced by carbon vacancies and the rise of the density of states with either temperature or the concentration of vacancies.

Key role of the cation interstitial structure in the radiation resistance of pyrochlores.

The annealing of the B cation interstitial is shown to drive the thermokinetic of the response to irradiations of A(2)B(2)O(7) pyrochlores. Molecular dynamics simulations evidenced that the annealing of interstitials created by irradiations depends upon the nature of B. As the coordination number of B decreases, the dumbbell interstitial is stabilized at the expense of the isolated interstitial. Unlike the isolated interstitials, the recombination of the dumbbells is thermally activated and hindered at low temperatures. The occurrence of dumbbells drives the structure towards the amorphous state.