Uranium is one of the most widely used elements among actinides, while its P-T phase diagram is still unknown. In this work, we present P-T phase diagram of uranium calculated up to extreme conditions. First, we searched for possible crystal structures using the evolutionary algorithm USPEX. Their free energies were then calculated using thermodynamic integration (TI) technique. TI was performed using molecular dynamics (MD), employing a machine learning (ML) forcefield trained on energies and forces from density-functional calculations at the generalized gradient approximation level. The prediction error of the ML forcefield for the energy was less than 10 meV/atom. Using thermodynamic perturbation theory (including 1st and 2nd order corrections), from the free energies of the ML forcefield, we obtained DFT free energies and DFT-quality phase diagram of uranium at pressures up to 800 GPa and temperatures up to 16000 K (see Fig. 1).

Fig. 1. Calculated P-T phase diagram of uranium.

Besides uranium, we also studied its hydrides at high pressures due to to the recent theoretical and then experimental discovery of record high-temperature superconductivity in H-S and La-H systems. Chemistry of the U-H system turned out to be extremely rich, with 14 new compounds, including hydrogen-rich UH₅, UH₆, U₂H₁₃, UH₇, UH₈, U₂H₁₇, and UH₉. Their crystal structures are based on either common face-centered cubic or hexagonal close-packed uranium sublattice and unusual H₈ cubic clusters. Our high-pressure experiments at 1 to 103 GPa confirm the predicted UH₇, UH₈, and three different phases of UH₅, raising confidence about predictions of the other phases. Many of the newly predicted phases are expected to be high-temperature superconductors. The highest-Tc superconductor is UH₇, predicted to be thermodynamically stable at pressures above 22 GPa (with Tc = 44 to 54 K), and this phase remains dynamically stable upon decompression to zero pressure (where it has Tc = 57 to 66 K).