Impact of longitudinal spin fluctuations on the structure of stable iron phase under extreme conditions*

Recent experiments provide the evidence of the body-centered cubic (bcc) iron at the submelting temperatures at 

extreme pressures. At the same time, there is no consensus among theoretical studies on the stability of the bcc phase.

All of those studies, however, agree that the free energies of the bcc and hcp (hexagonal close-packed) iron phases are extremely close

in the submelting range of temperatures at high pressures. Recently, two papers reported calculations of the bcc-fcc, fcc-bcc, and bcc-liquid

transition temperatures at 1 bar. Account for spin dynamics allowed to calculate the temperatures in very good agreement with experiment.

Therefore, it seems only logical to consider impact of magnetic entropy on the calculations of iron phase diagram under extreme conditions.

We performed density functional theory (DFT) based molecular dynamics (MD) simulations  taking into account longitudinal spin fluctuations

(LSF)  and discovered that the LSF account strongly affects properties of iron phases. We shall report phase diagram and properties of iron under

conditions of the Earth core and beyond using direct DFT-LSF MD simulations as well as the MD simulations with the machine learning potential

created using the data from the DFT-LSF MD runs.