Actinide dioxides derived from the AO 2 fluorite lattice are of high technological relevance due to their application in nuclear reactor fuels. In this paper we use density functional theory calculations to study the oxidation of uranium, neptunium and plutonium dioxides, AO 2 ͑A = U, Np, or Pu͒, in O 2 and O 2 / H 2 O environments. We pay particular attention to the formation of oxygen clusters ͑cooperativity͒ in AO 2+x and how this phenomenon governs oxidation thermodynamics and the development of ordered A 4 O 9 compounds. The socalled split di-interstitial, composed of two nearest-neighbor octahedral oxygen interstitials that dislocate one regular fluorite lattice oxygen ion to form a cluster of triangular geometry, is predicted to be the fundamental building block of the most stable cluster configurations. We also identify how the formation of oxygen defect clusters and the degree of oxidation in AO 2+x are both governed by the ability of the O 2p orbitals of the interstitial-like ͑+x͒ ions to hybridize with regular fluorite lattice ions.

Using density functional theory, we examine a recently discovered structure for di-interstitial oxygen clusters in UO 2+x in which three oxygen ions share one lattice site. This di-interstitial cluster exhibits a fast diffusion pathway; the migration barrier for these clusters is approximately half of that for mono-interstitials. Using kinetic Monte Carlo, we calculate the diffusivity of oxygen with and without the di-interstitial mechanism as a function of x and find that oxygen transport is significantly increased for higher values of x when the di-interstitial mechanism is included, agreeing much more closely with experimental data. These results emphasize the importance of clustering phenomena in UO 2+x and have implications for the evolution of UO 2+x .

No abstract

Uranium (U) exhibits a high temperature body-centered cubic (bcc) allotrope that is often stabilized by alloying with transition metals such as Zr, Mo, and Nb for technological applications. One such application involves U-Zr as nuclear fuel, where radiation damage and diffusion (processes heavily dependent on point defects) are of vital importance. Several systems of U are examined within a density functional theory framework utilizing projector augmented wave pseudopotentials. Two separate generalized gradient approximations of the exchange-correlation are used to calculate defect properties and are compared. The bulk modulus, the lattice constant, and the Birch-Murnaghan equation of state for the defect free bcc uranium allotrope are calculated. Defect parameters calculated include energies of formation of vacancies in the α and γ allotropes, as well as self-interstitials, Zr interstitials, and Zr substitutional defects for the γ allotrope. The results for vacancies agree very well with experimental and previous computational studies. The most probable self-interstitial site in γ-U is the (110) dumbbell, and the most probable defect location for dilute Zr in γ-U is the substitutional site. This is the first detailed study of self-defects in the bcc allotrope of U and also the first comprehensive study of dilute Zr defects in γ-U.

The detrimental effects of hydrogen and helium on structural materials undergoing irradiation are well documented, if not well understood. There is experimental evidence to suggest that a synergistic effect between the two elements exists, which results in increased damage when both are present. This situation is expected in the next generation of fusion and fission reactors, so a fundamental understanding of these synergistic interactions is needed to predict materials performance. We perform atomistic simulations of hydrogen and helium bubbles in body-centered cubic iron to determine the mechanism behind this effect. We first develop an interatomic potential suitable for describing the interactions between hydrogen and helium. Through analysis of the energetics and structure of these bubbles, we explain the observed synergy as a consequence of bubble growth through helium induced loop punching, aided by the presence of hydrogen, instead of as a direct interaction between hydrogen and helium. The hydrogen benefits from an increased area of free surface on which to bind.

The properties of the body-centered cubic γ phase of uranium (U) are calculated using atomistic simulations. First, a modified embedded-atom method interatomic potential is developed for the high temperature body-centered cubic (γ) phase of U. This phase is stable only at high temperatures and is thus relatively inaccessible to first principles calculations and room temperature experiments. Using this potential, equilibrium volume and elastic constants are calculated at 0 K and found to be in close agreement with previous first principles calculations. Further, the melting point, heat capacity, enthalpy of fusion, thermal expansion and volume change upon melting are calculated and found to be in reasonable agreement with experiment. The low temperature mechanical instability of γ U is correctly predicted and investigated as a function of pressure. The mechanical instability is suppressed at pressures greater than 17.2 GPa. The vacancy formation energy is analyzed as a function of pressure and shows a linear trend, allowing for the calculation of the extrapolated zero pressure vacancy formation energy. Finally, the self-defect formation energy is analyzed as a function of temperature. This is the first atomistic calculation of γ U properties above 0 K with interatomic potentials.

We predicted the ferroelectric-ferromagnetic multiferroic properties of EuTiO 3 nanowires and generated the phase diagrams in coordinates of temperature and wire radii. The calculations were performed within the Landau-Ginzburg-Devonshire theory with phenomenological parameters extracted from tabulated experimental data and first principles calculations. Since bulk EuTiO 3 is antiferromagnetic at temperatures lower than 5.5 K and paraelectric at all temperatures, our goal was to investigate the possibility of inducing the ferroelectric and ferromagnetic properties of EuTiO 3 by reducing the bulk to nanosystems.Our results indicate that ferroelectric spontaneous polarization of ~0.1-0.5C/m 2 is induced in EuTiO 3 nanowires due to the intrinsic surface stress, which is inversely proportional to the nanowire radius. The spontaneous polarization exists at temperatures lower than 300 K, for the wire radius less than 1 nm and typical surface stress coefficients ~ 15 N/m. Due to the strong biquadratic magnetoelectric coupling, the spontaneous polarization in turn induces the ferromagnetic phase at temperatures lower than 30 K for 2 nm nanowire, and at temperatures lower than 10 K for 4 nm nanowire in EuTiO 3 . Thus we predicted that the EuTiO 3 nanowires can be the new ferroelectricferromagnetic multiferroic.

Using Landau-Ginzburg-Devonshire theory, we have addressed the complex interplay between structural antiferrodistortive order parameter (oxygen octahedron rotations), polarization and magnetization in EuxSr1−xTiO3 nanosystems. We have calculated the phase diagrams of EuxSr1−xTiO3 bulk, nanotubes and nanowires, which include the antiferrodistortive, ferroelectric, ferromagnetic, and antiferromagnetic phases. For EuxSr1−xTiO3 nanosystems, our calculations show the presence of antiferrodistortive-ferroelectric-ferromagnetic phase or the triple phase at low temperatures (≤10 K). The polarization and magnetization values in the triple phase are calculated to be relatively high (∼50 μC/cm2 and ∼0.5 MA/m). Therefore, the strong coupling between structural distortions, polarization, and magnetization suggests the EuxSr1−xTiO3 nanosystems as strong candidates for possible multiferroic applications.