Structures and Stability of Iron Halides at the Earth’s Mantle and Core Pressures: Implications for the Missing Halogen Paradox

• Published in: ACS earth and space chemistry 2018-07, Vol.2 (7), p.711-719
• Main Authors: Du, XiangPo, Wang, Ziwei, Wang, Hongbo, Iitaka, Toshiaki, Pan, Yuanming, Wang, Hui, Tse, John S
• Format: Article
• Language: English
• ISSN: 2472-3452; 2472-3452
• DOI: 10.1021/acsearthspacechem.8b00034

The terrestrial abundance of heavy halogens Cl, Br, and I is depleted by approximately one order of magnitude relative to those predicted on the basis of their volatilities. One plausible explanation for this missing halogen paradox is their sequestration into the Earth’s core. Therefore, heavy halogens in the core may combine with the dominant element, Fe, to form iron halides that potentially exert important effects on the properties and dynamic evolution of the Earth’s inner core. In this study, stable iron halide phases have been predicted from first-principles structural searches at four pressures corresponding to those at the Earth’s mantle and core. At 360 GPa (corresponding to the inner core), the most stable iron chloride is CsCl-type FeCl, supporting the hypothesis that light-element impurities can stabilize the body-centered cubic Fe structure. At pressures of the Earth’s core, it is also observed that the chemical nature of iodine changes from an electron acceptor to an electron donor. This change results in an enhancement of the stability and the formation of a novel Fe2I compound containing a Fe–I framework with linear Fe chains intercalated in the open channels. Thus, the role of pressure in determining the stoichiometry of stable high-pressure halides is demonstrated by our theoretical calculations. These findings suggest the possibility of thermodynamic stability of iron halides in the assemblage in the Earth’s inner core.