The phase diagram of cobalt at high pressure and temperature:: the stability of γ(fcc)-cobalt and new ε′(dhcp)-cobalt

A metastable dhcp phase of cobalt, epsilon'-Co, 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 59.01 cm(3) mol(-1) with c/a = 3.190. First-principles theory shows that epsilon'(dhcp)-Co and epsilon(hcp)-Co are close in energy below 60 GPa, and that temperature and magnetic effects can make dhcp-Co more stable than hcp-Co. The gamma(fcc)-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.