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

Hydrogen-rich hydrides attract great attention due to recent theoretical (1) and then experimental discovery of record high-temperature superconductivity in H(3)S [T(c) = 203 K at 155 GPa (2)]. 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 UH(5), UH(6), U(2)H(13), UH(7), UH(8), U(2)H(17), and UH(9). Their crystal structures are based on either common face-centered cubic or hexagonal close-packed uranium sublattice and unusual H(8) cubic clusters. Our high-pressure experiments at 1 to 103 GPa confirm the predicted UH(7), UH(8), and three different phases of UH(5), raising confidence about predictions of the other phases. Many of the newly predicted phases are expected to be high-temperature superconductors. The highest-T(c) superconductor is UH(7), predicted to be thermodynamically stable at pressures above 22 GPa (with T(c) = 44 to 54 K), and this phase remains dynamically stable upon decompression to zero pressure (where it has T(c) = 57 to 66 K).