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Uranium is one of the main heat sources in the Earth, as about 25% of the total heat is produced by the radioactive decay of U. The location of U in the deep mantle is then essential for a better understanding of the geodynamics and thermal behavior of the Earth. For the first time, the crystal structure of natural simple dioxide U[O.sub.2] uraninite has been studied by X-ray diffraction with synchrotron radiation (ESRF, Grenoble, France), in situ in a laser-heated diamond-anvil cell at pressures and temperatures relevant to the deep Earth's mantle. Fluorite-type U[O.sub.2] displays a new sequence of phase transitions at high P and T, with a cubic modified fluorite Pa[bar.3] observed at 18 GPa, and an orthorhombic Pbca structure from 33 GPa up to 82 GPa. Using a second-order Birch-Murnaghan equation of state, we calculated room-pressure bulk modulus [K.sub.0] = 166(7) GPa with pressure derivative [K'.sub.0] = 4.0 for the Pa[bar.3] structure, and [K.sub.0] = 225(8) GPa with [K'.sub.0] = 4 for the Pbca structure. The expected Pnma cotunnite structure was not observed but is not excluded at pressures higher than 82 GPa. Since U[O.sub.2] displays a Pbca structure stable up to 82 GPa and presents a density much higher than the average density of the surrounding mantle, U[O.sub.2] could be a host of U in the deep lower mantle. Keywords: Heat sources, uranium oxide, X-ray diffraction, crystal structure, deep mantle

Two different approaches of the density functional theory are utilized to study the reactions of the uranium monoxide cations, U[O.sup.+] and U[O.sup.2+] with water, presenting a complete analysis of the gas-phase chemistry of actinide ions. The study, in combination with various topological methods, provides full description of the nature of the chemical bonding of the key minima and transition states, which can be further used for higher studies.

The complex formation of uranium(VI) with four nucleotides, adenosine (AMP), guanosine-(GMP), uridine-UMP) and cytidine-monophosphate (CMP) are studied in the alkaline pH range by (super 1)H, (super 31)P, (super 13)C, and (super 17) O NMR spectroscopy. The results have important implications on the synthesis of ologonucleotides.

Organometallic actinide bis(ketimide) complexes (C5Me5)2An[-N=C(Ph)(R)]2 of thorium(IV) and uranium(IV) is synthesized. Examination of the ground-state electronic structure, a detailed modeling of the photoelectron spectra, reveals similar bonding interactions between the thorium and uranium compounds.

Sterically crowded isoelectronic nf(super 3) (C5Me5)(sub 3)M complexes of neodymium and uranium, which are compounds having very long metal ligand distances, are studied for examining their reactivity with CO as a substrate. It is observed that in their isomorphous (C5Me5)(sub 3)M complexes, different reaction chemistry with CO is exhibited by the isoelectronic 4f(super 3) and 5f(super 5) Nd(III) and U(III) centers.

The low-temperature magnetic structure of UPdGe, which crystallizes in the orthorhombic TiNiSi structure, is reported. It was found that at 5 K, the uranium moments were about 1.5 mu(sub B)/U atom and unlike in UPdGe, it was seen that the magnetic moments were perpendicular to the shortest interuranium distance d(sub U-U).

The synthesis and structural analysis of a new organically templated uranyl zinc phosphate, [[[H.sub.2]bipy].sub.2][[(U[O.sub.2]).sub.6][Zn.sub.2][(P[O.sub.3]OH).sub.4][(P[O.sub.4]).sub.4]].[H.sub.2]O. (UZnP-2) is described. The structure consists of U[O.sub.7] pentagonal bipyramids that are linked into edge-sharing dimers that are joined together by Zn[O.sub.4] and P[O.sub.4] tetrahedra.

Sterically pressured mid- to high-valent uranium complexes with an aryloxide substituted triazacyclononane ligand scaffold are analyzed for carbon dioxide activation and transformation chemistry. The molecular structures of the synthesized uranium carbamate complexes have highlighted the different reactivities due to the steric pressure introduced by the alkyl derivatized tris(aryloxide) triazacyclononane ligand.

Hydrothermal methods are used for preparing four two-dimensional (2D) uranium(VI) carboxyphosphonates and their crystal structures are determined by single-crystal X-ray diffraction. Hydrogen bonding networks have played a crucial role in the formation and stabilization of the various structure types.

Several computational and experimental studies are conducted to explain the formation of several new thorium (Th) and uranium (U) ethylidine diydride, metallocyclopropane dihydride and vinyl metal trihydride complexes. The agostic interactions taking place in these systems are shown to lead to the distortion of these simple ethylidene complexes.

The synthesis and characterization of lanthanum(III) and uranyl(VI) complexes coordinated by tridentate diglycolamide ligands is described. The results for coordination spheres of the complexes might provide insight into the origin of some of the anomalous behavior observed for these metals in seperation studies.

The reactivity of the [[U[([N.sup.t]Bu).sub.2]].sup.2+] ion toward aryl isocyanates is examined. An unexpected reaction is observed that does not involve U[double bond]O bond formation but rather an imido exchange reaction in which aryl-imido for alkyl-imido substitution has occured.

A general and versatile Cu-(I)-based oxidation procedure that enables facile access to pentavalent organouranium complexes is described. It gives insight into derivatives electronic structure and bonding and also oxidation states of copper and uranium reagents.

The synthesis of a mixed-valent bisferrocene compound which supports a uranium-mediated electronic communication between the two iron centers is presented.

The synthesis of a uranium oxo imido complex, namely, trans-[U(NR)(O).sup.2+], which provides a link between an isostructural uranyl analogue and bis(imido) extremes, is reported. The structural data and theoretical results demonstrate that, like the bis(imido) complexes, the mixed oxo imido species still maintain many of the qualities that uranyl possesses, including significant uranium f and d electron participation in the multiple bonds.

Extended X-ray fine absorption structure (EXAFS) data on [U.sub.4][O.sub.9] were interpreted as showing that oxygen atoms are incorporated as oxo groups with a U-O distance of 1-72-1.76 Angstroms and that the uranium sublattice consists of an ordered portion and an additional 'spectroscopically silent' glassy portion. Powdered [U.sub.4][O.sub.9] neutron diffraction studies of the system however contradict these conclusions, showing that there are no U-O bonds shorter than 2.2 Angstroms and that [U.sub.4][O.sub.9] is crystallographically ordered with no evidence of a glassy structure.

A constitutive model that includes the effects of elasticity, crystal reorientation, phase transformations and plasticity is applied to a uranium- 6-wt% niobium (U-6Nb) alloy, which exhibits the shape memory effect. Computational simulations have demonstrated the ability of the model to capture the relevant physical response of the material.

Extended X-ray absorption fine structure (EXAFS) is utilized to investigate the local atomic structure around Th, U and Pu atoms in polycrystalline mixed dioxides [Th.sub.1-x][M.sub.x][O.sub.2] (with M = U, Pu) for x ranging from 0 to 1. The weighted mean cation-anion and cation-cation bond lengths determined from EXAFS data were showing high accuracy.

The synthesis, structural and spectroscopic properties of a series of uranium(IV)-perrhenato complexes with phosphate based and phosphine-oxide-based ligands are described. The structural characterization of all these complexes reveal eight-coordinate U(super IV) centers.

The synthesis, structure and spectroscopic properties of Am(IO3)(sub 3) and the photoluminescence data of Cm(IO3)(sub 3) are reported. The electronic levels of the actinide cations are based on the photoluminescence measurements and they show the presence of vibronic coupling between electronic transitions and IO(sub 3)(super -) vibrational modes in both compounds.

Ab initio methods are applied for the calculation of the total energy and the molar entropy and heat capacity of the compounds UO(sub 2)F(sub 2), UO(sub 2)(OH(sub 2)), UF(sub 6), and UO(sub 3) in the gas phase to obtain thermodynamic data for reactions that are comparable with experimental values. The transition state of UO(sub 2)F2(g) is studied and the value of the barrier for the inversion of the fluoride atoms are calculated.

The electronic and geometric structures of the analogues of the uranyl ion containing the -N=U=N- unit are examined using the quasi-relativistic gradient-corrected density functional theory. The results lead to the conclusion that the U-N bonding in UON(super +) and UN2 is significantly more covalent than the U-O bonding in UON(super +) and UO2(super +).

Nuclear reactions allow us to create elements beyond uranium (element 92), the heaviest element in nature. However, we are far from knowing what the heaviest possible element is. In the [...]