Uranium polyhydrides at moderate pressures: Prediction, synthesis, and expected superconductivity

Abstract

Hydrogen-rich hydrides attract great attention due to recent theoretical (<i>1</i>) and then experimental discovery of record high-temperature superconductivity in H₃S [<i>T</i> _c = 203 K at 155 GPa (<i>2</i>)]. Here we search for stable uranium hydrides at pressures up to 500 GPa using ab initio evolutionary crystal structure prediction. 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-<i>T</i> _c superconductor is UH₇, predicted to be thermodynamically stable at pressures above 22 GPa (with <i>T</i> _c = 44 to 54 K), and this phase remains dynamically stable upon decompression to zero pressure (where it has <i>T</i> _c = 57 to 66 K).