Stability and anisotropy of (FexNi1−x)2O under high pressure and implications in Earth’s and super-Earths’ core

Abstract

Oxygen is thought to be an important light element in Earth’s core but the amount of oxygen in Earth’s core remains elusive. In addition, iron-rich iron oxides are of great interest and significance in the field of geoscience and condensed matter physics. Here, static calculations based on density functional theory demonstrate that I4/mmm-Fe2O is dynamically and mechanically stable and becomes energetically favorable with respect to the assemblage of hcp-Fe and -FeO above 270 GPa, which indicates that I4/mmm-Fe2O can be a strong candidate phase for stable iron-rich iron oxides at high pressure, perhaps even at high temperature. The elasticity and anisotropy of I4/mmm-(FexNi1−x)2O at high pressures are also determined. Based on these results, we have derived the upper limit of oxygen to be 4.3 wt% in Earth’s lower outer core. On the other hand, I4/mmm-(FexNi1−x)2O with high AV S is likely to exist in a super-Earth’s or an ocean planet’s solid core causing the locally seismic heterogeneity. Our results not only give some clues to explore and synthesize novel iron-rich iron oxides but also shed light on the fundamental information of oxygen in the planetary core.

Equations of State and Anisotropy of Fe‐Ni‐Si Alloys

“…Concerning the stability of these phases in the inner core, Ozawa et al () find B2‐FeO to be favored over B1‐FeO above 240 GPa at 4000 K, thereby suggesting the B2 phase is stable in the inner core, and ab initio studies by Huang et al () propose I4/mmm ‐Fe 2 O is energetically favorable above 270 GPa. However, experimental equations of state are not presently available for B2‐FeO or I4/mmm ‐Fe 2 O.…”

“…The uncertainties on density, adiabatic bulk modulus, and bulk sound speed at high temperature for FeO, FeS 2 , and Fe 7 C 3 were not estimated as proper uncertainty propagation would require knowledge of the covariance matrix relating the fitted parameters V 0 , K T0 , K ′ T0 , 0 , and q. Concerning the stability of these phases in the inner core, Ozawa et al (2011) find B2-FeO to be favored over B1-FeO above 240 GPa at 4000 K, thereby suggesting the B2 phase is stable in the inner core, and ab initio studies by Huang et al (2018) Ahrens and Jeanloz (1987) suggest that cubic pyrite is stable up to 320 GPa. While recent theory suggests that C2/m-FeS 2 may be stable at inner core conditions (Bazhanova et al, 2017), experimental equations of state are also not presently available for this phase.…”

Iron and its compounds in the earth’s core: new data and ideas

“…Еще один необычный оксид (Fe x Ni 1-x ) 2 O недавно был предсказан на основании энергетических расчетов как устойчивый в интервале давлений 270-400 ГПа и, соответственно, как возможный компонент ядра Земли (Huang et al, 2018). Его структура изображена на рис.…”

Importance of the many-body effects for structural properties of the novel iron oxide: Fe$_2$O

“…Structural stability of different phases of Fe 2 O. Different structural phases of Fe 2 O were studied by Weerasinghe et al, and Huang et al using DFT calculations 1,23 . According to Ref.…”

“…1 Fe 2 O is formed at pressures ∼ 100 GPa. They found that below 180 GPa 1 (212 GPa 23 ) it has the P6 3 /mmc structure. Then it transforms to the P 3m1 phase and finally at 288 GPa 1 (266 GPa 23 ) to the I4/mmm structure.…”