PBE-GGA Predicts the B8$\leftrightarrow$B2 Phase Boundary of FeO at Earth's Core Conditions

TL;DR

This study uses ab initio calculations with PBE-GGA to accurately predict the phase boundary between B8 and B2 phases of FeO at Earth's core conditions, validating the method's effectiveness.

Contribution

It demonstrates that standard DFT functionals can reliably model FeO's phase transitions under extreme conditions, enabling predictive studies of Earth's core.

Findings

Reproduces experimental phase boundary within uncertainties at >240 GPa

Finds a negative Clapeyron slope of -52 ± 5 MPa/K

Validates PBE-GGA + Mermin functional for FeO under core conditions

Abstract

FeO is a crucial phase of the Earth's core, and its thermodynamic properties are essential to developing more accurate core models. It is also a notorious correlated insulator in the NaCl-type (B1) phase at ambient conditions. It undergoes two polymorphic transitions at 300 K before it becomes metallic in the NiAs-type (B8) structure at $\sim$100 GPa. Although its phase diagram is not fully mapped, it is well established that the B8 phase transforms to the CsCl-type (B2) phase at core pressures and temperatures. Here we report a successful \textit{ab initio} calculation of the B8$\leftrightarrow$B2 phase boundary in FeO at Earth's core pressures. We show that fully anharmonic free energies computed with the PBE-GGA + Mermin functional reproduce the experimental phase boundary within uncertainties at $P > 240$ GPa, including the largely negative Clapeyron slope of $-52 \pm 5$ MPa/K. This study validates the applicability of a standard DFT functional to FeO under Earth's core conditions and demonstrates the theoretical framework that enables complex predictive studies of this region.