Probing The 5f Electrons: A Relativistic DFT Study Of Americium Surfaces

Surface chemistry and physics have been and continues to be very active fields of research because of the obvious scientific and technological implications and consequent importance of such research. One of the many motivations for this burgeoning effort has been the desire to understand surface corrosion, metallurgy and catalytic activity in order to address environmental concerns. In particular, such efforts are important for a group of strongly correlated and heavy fermion systems like the actinides, for which experimental work is relatively difficult to perform due to material problems and toxicity. These metals are among the most complex of the long-lived elements, and in their solid state, they display some of the most unusual behaviors of any series in the periodic table, including very low melting temperatures, large anisotropic thermal expansion coefficients, very low symmetry crystal structures, and many solid-to-solid phase transitions. Radioactive and highly electropositive, the actinides are characterized by the 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 and are divided into two subgroups. The first subgroup consisting of Th to Pu, where the atomic volumes decrease with increasing 5f electron population, generally consists of delocalized 5f electrons. The second subgroup starting from Am onwards, shows increasing atomic volume with increasing 5f electrons, with the 5f electrons being localized. The open shell of the 5f electrons determines the magnetic and solid state properties of the actinide elements and their compounds. However, these properties of the actinides, particularly the transuranium actinides, are still not clearly understood. This stems primarily from the inherent difficulty in understanding the behavior of the 5f electrons, whose spatial extent and tendency to interact with electrons on ligand sites gives rise to the chemically complex nature of the transuranium actinides. The actinides are also characterized by the increasing prominence of relativistic effects and their study can, in fact, give us an in depth understanding of the role of relativity throughout the periodic table.
Among the transuranium actinides, the unique electronic properties of the manmade Americium (Am) metal, which was first successfully synthesized and isolated at the wartime Metallurgical Laboratory, have received increased interests, from both scientific and technological points of view. Am occupies a central position in the actinide series in our understanding of the behavior of the 5f electrons. It is widely believed that the properties and the behavior of the 5f electrons change dramatically starting from somewhere between Pu and Am. As a result, a large number of experimental and theoretical works have been done in recent years to gain insight into the structural and electronic properties of Am. In this work, atomic hydrogen and oxygen, molecular hydrogen and oxygen and water adsorptions on the (0001) surface of double hexagonal closed packed americium have been studied at both non-spin-orbit coupling (NSO) and spin-orbit coupling (SO) using generalized gradient approximation to density functional theory using the Perdew-Burke-Ernzerhof (PBE) formulation for the exchange-correlation functional. For atomic and molecular adsorptions, various chemisorption sites such as, top(t1), bridge(b2) and hcp(h3) have been investigated. Details of energetics of the chemisorption process, such as chemisorption energies, adatom/admolecule separation distances and inter molecular distances will be presented. Magnetic moments are also calculated for bare americium and the chemisorbed system. Adsorption of molecular hydrogen and oxygen and possible dissociative adsorption on americium surface will be presented. The adsorbate-substrate interactions have been analyzed in detail using the partial charges inside the muffin-tin spheres, difference charge density distributions, and the local density of states. The effects of adsorption on the Am 5f electron localization-delocalization characteristics will be conversed. The role of 5f electrons in the bonding of americium with the adatom/admolecule will be discussed.