Free energies of iron phases at high pressure and temperature: Molecular dynamics study

Anatoly B Belonoshko,Jie Fu,Grigory Smirnov

doi:10.1103/physrevb.104.104103

Abstract

The crystal structure of iron, the major component of the Earth's inner core (IC), is unknown under the IC high pressure (P) (3.3--3.6 Mbar) and temperature (T) (5000--7000 K). Experimental and theoretical data on the phase diagram of iron at these extreme PT conditions are contradictory. Applying quasi-ab initio and ab initio molecular dynamics we computed free energies of the body-centered cubic (bcc), hexagonal close-packed (hcp), and liquid phases. The ionic free energies, computed for the embedded-atom model, were corrected for electronic entropy. Such correction brings the melting temperatures of the hcp iron in very good agreement with previous ab initio data. This validates the calculation of the bcc phase, where fully ab initio treatment is not technically possible due to large sizes required for convergence. The resulting phase diagram shows stabilization of the bcc phase prior to melting in the pressure range of the IC. The melting temperature of the bcc phase is equal to 7190 K at the pressure 360 GPa.

Highlights

It has been established that the Earth’s inner core (IC) mainly consists of iron [1,2,3,4,5]
It was demonstrated that the thermodynamic stabilization of bcc on temperature increase at the pressure in the center of the Earth is due to electronic entropy [18]
That decreases the diffusion (Fig. 2). Such decrease might restrict the number of available states and, correspondingly, the entropy that is proportional to the logarithm of the visited states

Summary

Introduction

It has been established that the Earth’s inner core (IC) mainly consists of iron [1,2,3,4,5]. It has been assumed that Fe is stable at the IC pressure (P) (3.3–3.6 Mbar) and temperature (T) (5000–7000 K) in the hexagonal close-packed (hcp) phase, likely alloyed with Ni and light elements [5]. This assumption, has some ground—the hexagonal close-packed (hcp) phase of Fe was demonstrated to be stable at room temperature up to the pressure of 3 Mbar [5]. A number of phases have been suggested in addition to face-centered cubic (fcc), body-centered cubic (bcc), hcp, and liquid. The general consensus, based on the observation of the hcp [6,7,8,9] in a broad-PT range (but not quite at the IC PT conditions) is that the hcp phase is stable in the IC

Methods

Results

Discussion

Conclusion