• Donnelly, Mary-Ellen
• Dalladay-Simpson, Philip
• Binns, Jack
• Wang, Mengnan
• Howie, Ross T.
• Peña-Alvarez, Miriam

Abstract

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 (CoH<SUB>2</SUB>). 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 CoH<SUB>2</SUB>, where the hydrogen atoms occupy the tetrahedral vacancies. We have explored alternative synthesis routes of CoH<SUB>2</SUB> and can lower the synthesis pressure to 35 GPa by the application of high temperature. CoH<SUB>2</SUB> 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, CoH<SUB>2</SUB> has a high hydrogen content of 3.3 wt. % and a volumetric hydrogen density of 214 g/l....

Topics

• density
• impedance spectroscopy
• Hydrogen
• forming
• cobalt