On The Behavior Of The 5f Electrons

Considerable theoretical efforts have been devoted in recent years to studying the electronic and geometric structures and related properties of surfaces to high accuracy. One of the many motivations for this burgeoning effort has been a desire to understand the detailed mechanisms that lead to surface corrosion in the presence of environmental gases; a problem that is not only scientifically and technologically challenging but also environmentally important. Such efforts are particularly important for systems like the actinides for which experimental work is relatively difficult to perform due to material problems and toxicity. As is known, the actinides are characterized by a gradual filling of the 5f-electron shell with the degree of localization increasing with the atomic number Z along the last series of the periodic table. The open shell of the 5f electrons determines the magnetic and solid-state properties of the actinide elements and their compounds and understanding the quantum mechanics of the 5f electrons with increasing prominence of relativistic effects is the defining issue in the physics and chemistry of the actinide elements. The 5f orbitals have properties intermediate between those of localized 4f and delocalized 3d orbitals and as such, the actinides constitute the "missing link" between the d transition elements and the lanthanides. Among the actinide elements, uranium is well known due its use as a nuclear reactor fuel and is the heaviest naturally occurring actinide element. It is located in the middle of the early part of the actinide series, with only three 5f electrons hybridizing with the 6d and 7s electrons and demonstrating itinerant behavior. The proportion of the outer shell s and d electrons is larger in uranium compared to plutonium and a study of the electronic structure of U can provide significant clues about the crossover from delocalized to localized 5f-electron behavior supposed to occur somewhere in the region of the periodic table from uranium (with 3 5f electrons) to plutonium (5 5f electrons) to americium (with 6 5f electrons). Uranium crystallizes in the orthorhombic D-phase with four molecules per unit cell at ambient condition, followed by the body-centered tetragonal beta-phase at 940 K and then the body-centered  phase at 1050 K at ambient pressure. However, certain impurities like molybdenum can stabilize the -phase at room temperature or below. In this work, oxygen and carbon adsorptions on the (100) surface of γ-uranium have been studied at both non-spin-polarized and spin-polarized levels using the generalized gradient approximation of density functional theory (GGA-DFT) with Perdew and Wang (PW) functionals. For oxygen and carbon adsorption, various chemisorption sites such as, top, bridge, center, and interstitial have been investigated. Details of energetics of the chemisorption process, such as chemisorption energies, adatom separation distances, spin and charge distributions, energy band gaps and density of states will be presented. Magnetic moments are also calculated for bare uranium and oxygen and carbon adsorbed system. The changes in the uranium surface after adsorption of oxygen and carbon are analyzed and compared with the adsorption of atomic oxygen adsorption on the plutonium (100) surface. Also adsorption of molecular carbon monoxide and possible dissociative adsorption on uranium surface will be presented at both non-spin-polarized and spin-polarized levels of theory. For adsorption of carbon monoxide, different approaches such as Vert1, Vert2, Hor1 and Hor2 are studied at top, center, and bridge chemisorption sites. The role of 5f electrons in the bonding of uranium with the oxygen and carbon adatom, and with the carbon monoxide molecule will be discussed