Atomistic simulation of cubic and tetragonal phases of U-Mo alloy: Structure and thermodynamic properties

We studied structure and thermodynamic properties of cubic and tetragonal phases of pure uranium and U-Mo alloys using atomistic simulations: molecular dynamics and density functional theory. The main attention was paid to the metastable gamma(0)-phase that is formed in U-Mo alloys at low temperature. Structure of gamma(0)-phase is similar to body-centered tetragonal (bct) lattice with displacement of a central atom in the basic cell along [001] direction. Such displacements have opposite orientations for part of the neighbouring basic cells. In this case, such ordering of the displacements can be designated as antiferrodisplacement. Formation of such complex structure may be interpreted through forming of short U-U bonds. At heating, the tetragonal structure transforms into cubic gamma(s)-phase, still showing ordering of central atom displacements. With rise in temperature, gamma(s)-phase transforms to gamma-phase with a quasi body-centered cubic (q-bcc) lattice. The local positions of uranium atoms in gamma-phase correspond to gamma(s)-phase, however, orientations of the central atom displacements become disordered. Transition from gamma(0) to gamma can be considered as antiferro-to paraelastic transition of order-disorder type. This approach to the structure description of uranium alloy allows to explain a number of unusual features found in the experiments: anisotropy of lattice at low temperature; remarkably high selfdiffusion mobility in gamma-phase; decreasing of electrical resistivity at heating for some alloys. In addition, important part of this work is the development of new interatomic potential for U-Mo system made with taking into account details of studied structures. (C) 2017 Elsevier B.V. All rights reserved.