High Temperature And High Pressure Synthesis And Stability Study Of Transition Metal Hydrides

Studies of metal hydrides are often driven by the pursuit to produce a hydrogen-storage material that has high volumetric hydrogen density and high gravimetric content.High pressure has been an effective and clean route to producing stoichio-metric and non-stoichiometric hydrides with extremely high hydrogen contents.In situ high-pressure high-temperature X-ray powder diffraction studies of the cobalt-hydrogen system and nickel-hydrogen system reveal the direct synthesis of both the binary cobalt hydride(CoH),(CoH2)and nickel hydrides(Ni2H3).We observe the formation of fcc CoH at pressures of 4 GPa,which persists to pressures of 45 GPa.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.Further research shows:CoH2 has a high hydrogen content of 3.3 wt.%and a volumetric hydrogen density of 214 g/1.In the study of nickel hydrides,we observed the formation of NiH at pressure above 1 GPa,which remains stable to at least 52 GPa at room temperature.Laser heating to above 1000 K at this pressure we saw the formation of a body-centered monoclinic unit cell,which we indentified as Ni2H3.The new phase was observed to convert back to NiH below 25 GPa,and upon further decompression to atomopheric conditions,NiH slowly releases hydrogen with time.