Structure Evolutions of Iron Compounds under Pressure Show an Unusual Chemistry in Deep Earth

The terrestrial abundance of many elements, including heavy halogens Cl, Br, and I is approximately one order of magnitude lower than that predicted from their volatilities. One possible explanation is that these heavy elements are sequestered into the Earth’s core. This suggestion is supported by recent computational studies showing that heavy p elements may combine with iron at high pressure. Using first-principles electronic structure calculations and the automatic crystal structure search method based on particle swarm optimization algorithm, we also studied the stability and structures of Fe-halogen compounds under high pressure up to 350 GPa. Our calculations show that the compounds with higher Fe composition become more stable with increasing pressure and the reaction propensity of Fe might become opposite to ambient pressure. Our detailed electronic structure analysis reveals that the charge capture by Fe 3d orbitals and the reduction of the lone pair electrons in halogens are the major factors that govern the structure evolution under increasing pressure. Our results suggest that the distribution of many p-block elements in the Earth core might be much higher than we usually believe.