Computational searches for iron oxides at high pressures

Area of Science:

• Materials Science
• Computational Chemistry
• Geophysics

Background:

• Understanding the behavior of iron and oxygen under extreme pressure is crucial for planetary science and materials research.
• Previous studies have explored iron-oxide phase diagrams, but comprehensive predictions at multi-hundred GPa are limited.

Purpose of the Study:

• To predict stable structures and stoichiometries of iron-oxygen mixtures at high pressures (100-500 GPa).
• To identify novel high-pressure phases of iron oxides and compare their stability with known structures.

Main Methods:

• Density-functional theory (DFT) calculations were employed.
• The ab initio random structure searching (AIRSS) approach was utilized to explore a wide range of possible structures.
• Over 32,000 structures were relaxed across 12 different stoichiometries and three pressure points.

Main Results:

• Fe2O3 and FeO2 were found to be the only stable phases at 100 GPa.
• New stable or near-stable structures were identified for FeO, Fe3O4, and Fe4O5 at 350 and 500 GPa.
• A new P2(1)2(1)2(1)2-symmetric Fe2O3 phase is more stable than the Rh2O3(II) phase above ~233 GPa.
• Novel FeO, Fe3O4, and Fe4O5 structures with lower enthalpies than known phases were discovered within specific pressure ranges.

Conclusions:

• The high-pressure phase diagram of iron-oxygen systems is more complex than previously thought.
• The newly predicted structures offer insights into the behavior of materials under extreme conditions.
• This study provides a foundation for future experimental investigations of high-pressure iron oxides.