On the Ground State of the U-Mo System

The ground state of the U-Mo system is studied in this work using an evolutionary algorithm coupled with \textit{ab initio} calculations. This methodology is applied to several compositions, with a more detailed examination at the U-rich side. A new ground state is identified within an approach that considers structures for up to 3 formula units per cell. A supercell scheme based on the pure phase structure of each element is used for dilute compositions. The ground state at a pressure of 1 atm. is characterized by only two stable compounds: $\Omega$-phase and a new phase, named $\alpha$-U$_{15}$Mo in this work, with a structure determined by adding a substitutional Mo impurity to a $1\times 2\times 2$ supercell of $\alpha$-U. A range of immiscibility of U in bcc-Mo based structures is extended from 0 to nearby 50 at. \% of U. The new $\alpha$-U$_{15}$Mo phase comprising the ground state has similar features to those of the $\alpha'$-phase formed in quenched samples at room temperature. The internal relaxation of U atoms in this phase resembles the movements due to charge density waves observed in orthorhombic $\alpha_1$-U. The ground state of the U$_{15}$Mo compound at T=0 K can not be fully identified due to the charge density wave associated with $\alpha$-U. That structure could be a supercell based on the structurally undefined $\alpha_3$-U phase.