Phase diagram, mechanical and thermodynamics properties of metallic Co under high temperature and high pressure

•We present the phase diagram of metallic Co in theory for the first time.•The elastic constants of metallic Co under temperature are simulated firstly.•The harmonic characters of hcp Co at high-temperature are discussed.•The thermodynamic properties of hcp and fcc Co are predicted successfully.

The phase diagram, mechanical and thermodynamic properties of metallic Co are investigated extensively using first-principles calculations in conjunction with a quasi-harmonic Debye model. The simulations indicate that metallic Co is a ferromagnetic hcp (εε) structure at 0 K and 0 GPa, and transforms to a nonmagnetic fcc (ββ) structure at about 120 GPa under compression, the transition is consistent with the experimental results (crystallographic transition) and other theoretical calculations. Particularly, from the calculated Gibbs free energy, the phase boundary between the ferromagnetic hcp (εε) phase and the ferromagnetic fcc (γfγf) phase is drawn, it is close to the conjecture of Yoo et al. (2000) (the phase boundary between the ferromagnetic hcp phase (εε) and the mixed phases (ε+β/γε+β/γ) of the ferromagnetic hcp (εε) structure and the ferromagnetic or paramagnetic fcc (γγ), or nonmagnetic fcc (ββ) structure) as well as the part of experimental data under high temperature and high pressure. The elastic constants of the ferromagnetic hcp- and fcc-Co under temperature are presented for the fist time in theory, they are in reasonable agreement with experiments at zero temperature and zero pressure, and decrease slightly with increasing temperature at V/V0=1V/V0=1 or V/V0=0.97V/V0=0.97. In addition, the heat capacity CV,CV, thermal expansion coefficient αα and Grüneisen parameter γ for the ferromagnetic hcp- and fcc-Co are also acquired and predicted successfully in an extended temperature and pressure range.

Graphical abstractThe phase diagram of metallic Co under high temperature and pressure, εε means the ferromagnetic hcp phase of metallic Co, γγ means the ferromagnetic or paramagnetic fcc phase of metallic Co, ββ means the nonmagnetic fcc phase of metallic Co, γfγf means the ferromagnetic fcc phase of metallic Co.Figure optionsDownload full-size imageDownload as PowerPoint slide