  Hydrogen-rich hydrides attract great attention due to recent theoretical (1)
and then experimental discovery of record high-temperature superconductivity in
H3S (Tc = 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 UH5, UH6, U2H13, UH7, UH8, U2H17, and
UH9. Their crystal structures are based on either common f.c.c. or h.c.p.
uranium sublattice and unusual H8 cubic clusters. Our high-pressure experiments
at 1-103 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 = 44-54 K) and this phase remains dynamically
stable upon decompression to zero pressure (with Tc = 57-66 K).
