Atomistic simulations of Ga atom ordering in Pu 5 at. % Ga alloys

We employ molecular dynamics and Monte Carlo (MC) methods to simulate the rearrangement of Ga atoms from a randomly distributed Pu-Ga alloy and study the resultant effect on thermodynamic properties. The results show that all of the first neighbor Ga-Ga bonds are removed at all temperatures considered (200, 400 and 600 K) while the number of 2NN and 3NN bonds increase, and the number of 4NN bonds decreases. These results imply that Ga atoms develop strong short range ordering in the solid solution. The ordering causes an enthalpy decrease about similar to 3-4 meV/atom for different temperatures in the 5 at. % Ga alloy. This energy change is clearly important in the calculation of the Pu-Ga phase diagram. In addition, MC calculations at 200 K show pronounced Ga segregation.