Synthesis of UH7 and UH8 superhydrides: Additive-volume alloys of uranium and atomic metal hydrogen down to 35 GPa

UH7 and UH8 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 GPa. The sequence UH3, UH7, and UH8 is observed under increasing pressure using synchrotron x-ray diffraction. UH7 is stable above 35 GPa and UH8 above 87 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, UH7 is the superhydride for which the concept of chemical precompression to stabilize an atomic metal hydrogen analog works the most effectively so far.