High pressure synthesis and stability of cobalt hydrides

In situ high-pressure high-temperature X-ray powder diffraction studies of the cobalt-hydrogen system reveal the direct synthesis of both the binary cobalt hydride (CoH) and a novel cobalt dihydride (CoH₂). We observe the formation of fcc CoH at pressures of 4 GPa, which persists to pressures of 45 GPa. At this pressure, we see the emergence with time of a further expanded fcc lattice, which we identify as CoH₂, where the hydrogen atoms occupy the tetrahedral vacancies. We have explored alternative synthesis routes of CoH₂ and can lower the synthesis pressure to 35 GPa by the application of high temperature. CoH₂ is stable to at least 55 GPa and decomposes into CoH below 10 GPa, releasing molecular hydrogen before further decomposing completely into its constituent elements below 3 GPa. As a first-row transition metal, cobalt has a relatively lower mass than other hydride-forming transition metals, and as a result, CoH₂ has a high hydrogen content of 3.3 wt. % and a volumetric hydrogen density of 214 g/l.