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: $Ω$-phase and a new phase, named $α$-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 $α$-U. A range of immiscibility of U in bcc-Mo based structures is extended from 0 to nearby 50 at. \% of U. The new $α$-U$₁₅$Mo phase comprising the ground state has similar features to those of the $α'$-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 $α_1$-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 $α$-U. That structure could be a supercell based on the structurally undefined $α_3$-U phase.