The stability and the structures of Fe-I and Fe-Br compounds under Earth core condition

The reactivity of Iron with non-metals is important for the steel industry as well as for understanding the Earth core compositions and properties. For latter case, it is essential to understand the change of chemical propensity of Fe under high pressure. Using first principles electronic structure calculations and the automatic crystal structure search method based on particle swarm optimization algorithm, we studied the stability and structures of Fe-I and Fe-Br compounds under high pressure up to 350 GPa. Our calculations show that the compounds with higher Fe composition become more stable with increasing pressure. Similar stability trend is also shown for Fe-Br compounds. We also found that Fe forms more stable compounds with I than Br, which is opposite to the chemical propensity at ambient pressure. These results suggest that the I/Br distribution ratio the Earth core might be much higher than we usually believe.