Phase diagram of iron under the Earth Core pressures

The Earth core consists mainly of iron subjected to pressures from 1.3 to 3.6 Mbar and temperatures from about 3000 K to 7000 K.

While pressures are known rather precisely, the temperature range is less known. The phase diagram of iron allows to set 

tight constraints on the temperature within the Earth and provide interpretaion of seismic data. We used a combination of 

quasi ab initio embedded-atom model and density functional theory to perform molecular dynamic calculations of free energies of

liquid, hexagonal close packed (hcp) and body-centered cubic (bcc) iron phases. These calculations tell us that in the pressure range between 1.2 and 3.6

Mbar the sequence of iron phases on increasing temperature is hcp-bcc-liquid. This is in agreement with some experimental data, yet

there are experiments that do not observe the bcc phase. However, our phase diagram is a perfect match to the latest seismic data that rules

out the hcp phase as irrelevant for the Earth Inner Core mineralogy. Namely, that seismic study tells us that the directions of fast and slow

propagation of sound in the Earth Inner Core cross at the angle of 54 degrees. The same angle in the bcc phase (directions 100 and 111) is equal

to 54.73 degrees. There is no such angle in the hcp phase. This is an ultimate evidence for the bcc stability in the Inner Core.

We also demonstrate that all previous calculations of free energies of the bcc phase are inconclusive since they have been performed for too small computational cells.

We demonstarted that cells with at least 1000 atoms are needed to get converged results, Since the iron in the Inner Core can contribute to the

Earth-protecting magnetic field, our result has long reaching consequences for magnetic field generation both on Earth and exoplanets.