Models of Mixed Metal-Oxide Interfaces for Atomistic Materials Simulations
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Models of Mixed Metal-Oxide Interfaces for Atomistic Materials Simulations Steven M. Valone1 1 Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A. LA-UR-12-20829 25 Apr 2012 ABSTRACT Nuclear fuels and materials present special problems to atomistic-scale modeling. At a metal-metal-oxide interface, the metal centers are charged on the oxide side, but neutral on the metallic side. The intimate contact necessitates that atomistic models for these materials be both compatible and consistent with one another at some level. A new "fragment'' Hamiltonian (FH) model, at the atomistic level, is presented that reduces qualitatively to existing, successful models for metals, such as the embedded atom method, and ceramics, such as the charge equilibration models. Moreover, the FH model possesses both electron hopping and fundamental gaps that appear as separate terms in a generalized embedding function. The electron hopping contributions come from both one-electron and two-electron sources. These contributions appear as a result of the FH point of view, rather than being postulated. The model obeys certain wellknown theoretical limits that come from the nonlinearity of electron hopping processes as the volume of a crystal is changed. The generalized notion of embedding entails two variables instead of one. The ability to account for multiple charge states in the cations leads to the capability within the model to distinguish the qualitative differences among metallic, ionic, and covalent bonding environments. The details of all of these energies, among with fragmentfragment interactions, combine to determine the state of the atom in the material. INTRODUCTION The demands of nuclear fuel modeling place metal oxides in contact with metals and alloys at both the fuel-cladding interface and within oxide-dispersion strengthened steels. The intimate contact necessitates that atomistic models for these materials be both compatible and consistent with one another at some level. Presently, most atomistic models address only metals, or only ceramics, but rarely both. Those models that are able to address both types of materials do so, for the most part, without reducing properly to a known model for metals. In addition, cations in many actinide oxides themselves enter multiple oxidation states, depending on the composition. The most essential examples are those materials that can exist as either sesquioxides or dioxides. Actinide metals themselves present special challenges to atomistic models because of strong electron correlation effects. In atomic models devised to date, there is no explicit provision for how to represent strong electron correlation effects. Thus, there is an essential need for atomistic models with a more fundamental basis. One such effort addressed phase ordering in Pu metal at the level of the modified embedded atom method (MEAM) [1,2]. There the correct phase ordering between fcc and hcp phases was enforced. As secondary effects of that enforcement, other materia
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