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

Journal of Nuclear Materials. 2018. Vol. 499. P. 451–463.

Starikov S. V., Kolotova L., Kuksin A. Y., Smirnova D. E., Tseplyaev V. I.

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 g0-phase that is formed in U-Mo alloys at low temperature.

Structure of g0-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 gs-phase, still showing ordering of

central atom displacements. With rise in temperature, gs-phase transforms to g-phase with a quasi

body-centered cubic (q-bcc) lattice. The local positions of uranium atoms in g-phase correspond to

gs-phase, however, orientations of the central atom displacements become disordered. Transition from

g0 to g 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 g-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.

Keywords: Phase transition, молекулярная динамика, uranium, molybdenum, молибден, molecular dynamics, уран, фазовый переход, сплавы урана, Uranium alloy, Interatomic potential, межатомный потенциал