THE PHASE-DIAGRAM OF COBALT AT HIGH-PRESSURE AND TEMPERATURE - THE STABILITY OF GAMMA(FCC)-COBALT AND NEW EPSILON'(DHCP)-COBALT

A metastable dhcp phase of cobalt, epsilon'-Co, has been discovered be low 60 GPa by using in situ x-ray diffraction and a laser-heated diamo nd-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 epsilo n(hcp)-Co are close in energy below 60 GPa, and that temperature and m agnetic effects can make dhcp-Co more stable than hcp-Co. The gamma(fc c)-phase is found to be stable and quenchable over a wide range of pre ssure and temperature. New constraints for the phase diagram of Co are presented.