The phase diagram of cobalt at high pressure and temperature: the stability of -cobalt and new -cobalt

A metastable dhcp phase of cobalt, , has been discovered below 60 GPa by using in situ x-ray diffraction and a laser-heated diamond-anvil cell. The volume at 34 GPa is with = 3.190. First-principles theory shows that and are close in energy below 60 GPa, and that temperature and magnetic effects can make dhcp-Co more stable than hcp-Co. The -phase is found to be stable and quenchable over a wide range of pressure and temperature. New constraints for the phase diagram of Co are presented.