We report a new phase transition in cobalt from the magnetic $\varepsilon$(hcp) to a $\beta$(fcc) phase, likely nonmagnetic, at 105 GPa. It occurs martensitically in an extended pressure region between 105 and 150 GPa without any apparent volume change. The fcc phase of Co is in systematic accordance with the $4d$ and $5d$ counterparts. The pressure-volume isotherm of $\beta$-Co resembles those of $\alpha$(fcc)-Ni and $\varepsilon$(hcp)-Fe within 1%. The phase diagram of cobalt suggests that the fcc stability increases with increasing occupancy of $d$-band electrons from Fe to Co to Ni.

• Received 16 November 1999

DOI:https://doi.org/10.1103/PhysRevLett.84.4132