${\mathrm{UH}}_7$ and ${\mathrm{UH}}_8$ have been synthesized in a pure form by direct reaction of uranium with excess hydrogen in a laser-heating diamond anvil cell up to $96\mathrm{GPa}$. The sequence ${\mathrm{UH}}_3$, ${\mathrm{UH}}_7$, and ${\mathrm{UH}}_8$ is observed under increasing pressure using synchrotron x-ray diffraction. ${\mathrm{UH}}_7$ is stable above $35\mathrm{GPa}$ and ${\mathrm{UH}}_8$ above $87\mathrm{GPa}$. The crystal structures of these two superhydrides correspond to the ab initio predicted structures with their cagelike sublattice of atomic hydrogen. Their volume expansion per H atom falls on the hypothetical atomic metal hydrogen compression curve. A critical survey of the literature data for other recently observed superhydrides enables one to generalize such a trend in the partial volume occupied by the hydrogen atom. Hence, superhydrides made only of atomic hydrogen can be considered additive-volume alloys of atomic metal hydrogen and the metal. With its lowest equilibrium pressure, ${\mathrm{UH}}_7$ is the superhydride for which the concept of chemical precompression to stabilize an atomic metal hydrogen analog works the most effectively so far.

• Received 12 April 2020
• Revised 7 June 2020
• Accepted 25 June 2020

DOI:https://doi.org/10.1103/PhysRevB.102.014107