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.

For 3-5d transition-metal ions, the (C5R5)2MCl2 (R = H, Me for M = Ti, Zr, Hf) bent metallocenes represent a series of compounds that have been central in the development of organometallic chemistry and homogeneous catalysis. Here, we evaluate how changes in the principal quantum number for the group IV (C5H5)2MCl2 (M = Ti, Zr, Hf; 1- 3, respectively) complexes affects the covalency of M-Cl bonds through application of Cl K-edge X-ray Absorption Spectroscopy (XAS). Spectra were recorded on solid samples dispersed as a thin film and encapsulated in polystyrene matrices to reliably minimize problems associated with X-ray self-absorption. The data show that XAS pre-edge intensities can be quantitatively reproduced when analytes are encapsulated in polystyrene. Cl K-edge XAS data show that covalency in M-Cl bonding changes in the order Ti > Zr > Hf and demonstrates that covalency slightly decreases with increasing principal quantum number in 1-3. The percent Cl 3p character was experimentally determined to be 26, 23, and 18% per M-Cl bond in the thin-film samples for 1-3 respectively and was indistinguishable from the polystyrene samples, which analyzed as 25, 25, and 19% for 1-3, respectively. To aid in interpretation of Cl K-edge XAS, 1-3 were also analyzed by ground-state and time-dependent density functional theory (TD-DFT) calculations. The calculated spectra and percent chlorine character are in close agreement with the experimental observations, and show 20, 18, and 17% Cl 3p character per M-Cl bond for 1-3, respectively. Polystyrene matrix encapsulation affords a convenient method to safely contain radioactive samples to extend our studies to include actinide elements, where both 5f and 6d orbitals are expected to play a role in M-Cl bonding and where transition assignments must rely on accurate theoretical calculations.

USA andersson @Ianl.gov RECEIVED DATE CORRESPONDING AUTHOR FOOTNOTEDespite the chemical and structural simplicity of MgB2' at 39 K this compound has the highest known Tc of any binary compound. Electron doping by substituting Al for Mg leads to decreasing Tc and the observed concentration dependent rate of decrease has been proposed to arise from the non-ideal character of MgB2-AIB2 solid solutions, which derives from the existence of an ordered M~.sAlo.sB2 compound. Heterogeneous nano-scale structure patterns in solid solutions have emerged as an important concept for complex materials, ranging from actinide alloys and oxides to high-temperature cuprate superconductors and mallganite-based materials exhibiting colossal magnetoresistivity. In this work we investigate the formation of structural heterogeneities in Mg 1 . ,AI.B2' which take the form of nano-scale AI-AI and AI-Mg domains of different geometry and size, using molecular statics/dynamics simulations and in particular we study the corresponding signatures in diffraction experiments. In order to undertake this task we first derive appropriate 1 Mg-AI-B semi-empirical potentials within the Modified Embedded Atom Method formalism.These potentials are also applied to explore the equilibrium Mg 1 _ x Al x B 2 phase diagram for 0 < x < 0.5. Additionally, density functional theory calculations were utilized to study the influence of heterogeneities on the electronic structure and charge distribution in Mg 1 _ x AI.B 2 • KEYWORDS

Equilibria E 4000Modeling of the Phase Evolution in Mg1-xAlxB2 (0 < x < 0.5) and Its Experimental Signatures. -(ANDERSSON, D. A.; CASILLAS, L.; BASKES, M. I.; LEZAMA, J. S.; CONRADSON*, S. D.; J.