High-Pressure Formation of Cobalt Polyhydrides: A First-Principle Study.

The high-pressure phase diagram, crystal structures, and electronic properties of cobalt hydrides are systematically investigated in the pressure range of 1 atm to 300 GPa by first-principle calculations. Except for the experimentally found CoH, two new cobalt polyhydrides CoH2 and CoH3 are discovered at 10 and 30 GPa, respectively. The crystal structure of CoH2 is determined to have cubic symmetry with the space group Fm3̅m and then transforms into the I4/mmm phase above 42 GPa. In addition, CoH3 with Pm3̅m is stable between 30 and 300 GPa, which can be used as a potential hydrogen storage material with a high volumetric hydrogen density of 425 g H2/L. All the cobalt polyhydrides exhibit metallic and ionic characteristics at high pressure. Furthermore, application of the Allen-Dynes-modified McMillan equation estimated no superconductivity for cobalt polyhydrides.