Yangzhong Li

A re-formulated and re-parameterized interatomic potential for uranium metal in the Charge-Optimized Many-Body (COMB) formalism is presented. Most physical properties of the orthorhombic α and bcc γ phases are accurately reproduced. In particular, this potential can reproduce the negative thermal expansion of the b axis in α-U while keeping this phase as the most stable phase at low temperatures, in accord with experiment. Most of the volume expansion in α-U by intrinsic defects is shown to come from the b axis, due to the formation of prismatic loops normal to this direction. Glide dislocation loops forming stacking faults are also observed. Structures of both loop types are analyzed. An expansion simulation is conducted and the results are verified by using the Norgett-Robinson-Torrens model. Rather than forming extended defect structures as in α-U, the γ phase forms only isolated defects and thus results in a much smaller and isotropic expansion.