Spin Crossover in Ferropericlase from First-Principles Molecular Dynamics

Ferropericlase, (Mg,Fe)O, is the second-most abundant mineral of Earth’s lower mantle. With increasing pressure, the Fe ions in the material begin to collapse from a magnetic to nonmagnetic spin state. We present a finite-temperature first-principles phase diagram of this spin crossover, finding a broad pressure range with coexisting magnetic and nonmagnetic ions due to favorable enthalpy of mixing of the two. Furthermore, we find the electrical conductivity of the mineral to reach semimetallic values inside Earth.

Figure 1

Our first-principles phase diagram of the spin crossover in ferropericlase. The black line is a geotherm from Ref. [39].

Figure 2

(a) The EOS of ferropericlase at static conditions compared to experimental data gathered at 300 K [9, 16, 40, 41, 42, 43]. We construct the total EOS by interpolating $V=V(P,f)$ linearly between the four spin phases $f$, where the EOS for each phase is a fit to the third-order Birch-Murnaghan EOS [44]. (b) Our static result for $f$ compared to XES and MSB data [8, 13, 15] as well as previous computational results [10] at 300 K. (c) Our thermal EOS for all simulated temperatures along with experiment at 2000 K [16].

Figure 3

Enthalpy (dashed lines) and internal energy (solid lines) of the mixing of high-spin and low-spin Fe at static conditions and the pressures indicated in the legend, along with the enthalpy of mixing at 2000 K and 91 and 64 GPa (green octagons, left to right, respectively), 3000 K and 98 and 73 GPa (orange pentagons), and 4000 K and 106 and 81 GPa (red squares). The lines are fits to $A(1-f{)}^{2}f+B(1-f)f^2$. Inset: Distribution of octahedral volumes at 2000 K and lattice constant $\lambda =3.93\AA$, which yielded $f\approx 0.50$ and $P=64\mathrm{GPa}$ (corresponding to the rightmost green octagon in the main figure), for Fe-O octahedra of high-spin Fe (blue) and low-spin Fe (red). The corresponding static results at the same $\lambda$ and $f$ are shown by the dashed vertical lines. For comparison, we also show the octahedral volume at static conditions and $\lambda =3.93\AA$ with homogeneous spin states: At the same volume, $f=1$ and $f=0$ produce identical values for the Fe-O octahedral volume (to within the thickness of the black dash-dotted line).