High-pressure phase transitions, electrical conductivity and recoverability of uranium polyhydrides

The uranium-hydrogen system has been predicted to form numerous polyhydride compounds, exhibiting diverse physical properties. However, experiments report starkly conflicting results regarding synthesis pressures, stoichiometries, and stability, leaving their fundamental properties and even their existence uncertain. Through a series of laser-heated diamond anvil cell experiments, we systematically investigate the high-pressure phase diagram of the uranium-hydrogen system up to 99(3) GPa using synchrotron powder X-ray diffraction. Compression and laser heating of β -UH 3 at pressures of 41(1) and 91(3) GPa result in the formation of hcp -UH 7 and fcc -UH 8 , respectively. Upon decompression of hcp -UH 7 below 27(1) GPa, we observe decomposition into hcp -UH 5 , which subsequently further dehydrogenates to U 4 H 15 below 12(1) GPa. The U 4 H 15 compound, analogous to the well-known Th 4 H 15 , is recoverable to ambient pressure and temperature, and slowly oxidizes over the timescale of hours. Whilst all compounds are metallic, electrical resistance measurements demonstrate an absence of superconductivity upon cooling to 3 K in U 4 H 15 , hcp -UH 5 , and hcp -UH 7 , and to 7 K in fcc -UH 8 .