Intermediate spin state and the B1-B2 transition in ferropericlase at tera-Pascal pressures

TL;DR

This study uses ab initio methods to explore the structure and magnetic transitions of ferropericlase at pressures up to 3 TPa, revealing a pressure-induced B1-B2 phase transition and spin state changes relevant to Earth's and super-Earths' mantles.

Contribution

It provides the first detailed ab initio analysis of ferropericlase's phase diagram and magnetic states at ultra-high pressures, including the identification of an intermediate spin state and its stability.

Findings

Ferropericlase undergoes a B1 to B2 phase transition at high pressure.

A pressure-induced change from low spin to intermediate spin state occurs.

The intermediate spin state is dynamically stable and influences mantle modeling.

Abstract

Ferropericlase (fp), (Mg$_{1-x}$Fe$_x$)O, the second most abundant mineral in the Earth's lower mantle, is expected to be an essential component of super-Earths' mantles. Here we present an ab initio investigation of the structure and magnetic ground state of fp up to $\sim$ 3 TPa with iron concentrations (x$_{Fe}$) varying from 0.03 to 0.12. Calculations were performed using LDA+U$_{sc}$ and PBE exchange-correlation functionals to elucidate the pressure range for which the Hubbard U (U$_{sc}$) is required. Similar to the end-members FeO and MgO, fp also undergoes a B1 to B2 phase transition that should be essential for modeling the structure and dynamics of super-Earths' mantles. This structural transition involves a simultaneous change in magnetic state from a low spin (LS) B1 phase with iron total spin S=0 to an intermediate spin (IS) B2 phase with S=1. This is a rare form of pressure/strain-induced magnetism produced by local cation coordination changes. Phonon calculations confirm the dynamical stability of the iron B2-IS state. Free energy calculations are then carried out including vibrational effects and electronic and magnetic entropy contributions. The phase diagram is then obtained for low concentration fp using a quasi-ideal solid solution model. For x$_{Fe}$ > 0.12 this approach is no longer valid. At ultra-high pressures, there is an IS to LS spin state change in Fe in the B2 phase, but the transition pressure depends sensitively on thermal electronic excitations and on x$_{Fe}$.