Structure Evolutions of Iron Halides under Pressure Show an Unusual Chemistry in Deep Earth
Abstract
The terrestrial abundance of heavy halogens Cl, Br, and I is approximately one order of magnitude lower than that predicted from their volatilities. One possible explanation for this "missing halogen paradox" is that these heavy halogens are sequestered into the Earth's core. This suggestion is supported by recent computational studies showing that heavy halogens may combine with iron, the dominant element of Earth's core, to form stable compounds at high pressure. The most stable iron chloride, for example, at the pressure of Earth's core (360 GPa), is a CsCl-type structure. 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. This change results in an enhancement of the stability and the formation of a novel Iron-halogen compounds such as Fe3I and Fe2I [1] containing a Fe-I framework with linear Fe chains intercalated in the open channels. 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.
[1] ACS Earth Space Chem. (2018) 2, 7, 711-719- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMMR43B0104F
- Keywords:
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- 3909 Elasticity and anelasticity;
- MINERAL PHYSICSDE: 3919 Equations of state;
- MINERAL PHYSICS