Magnetic and spin transitions in wüstite: A synchrotron Mössbauer spectroscopic study

This is a Mössbauer study of wüstite at pressures above 200 GPa using synchrotron Mössbauer spectroscopy. Synthetic $\mathrm{F}{\mathrm{e}}_{0.96}\mathrm{O}$-wüstite was investigated at 91(2), 95(4), 109(2), 114.5(3), 131.1(7), 133.2(2), 155(2), 167(2), 193(2) and 203(1) GPa at 300 K at the SPring-8 BL11XU beamline. The Mössbauer spectrum at 91 GPa consists of both magnetic and nonmagnetic components. The magnetic high-spin component decreases gradually with increasing pressure from 91 to 203 GPa, while the nonmagnetic low-spin component increases with pressure in the same pressure range. The result suggests that the spin state of Fe in the outer core at pressures above 203 GPa is the low-spin state. If oxygen exists in the core, the low-spin Fe-O bonding is shorter than high-spin Fe-O bonding, suggesting dense Fe-O liquid in the Earth's outer core. The gradual increase of the density of the metallic liquid with depth by the spin transition of Fe-O bonding in the shallow outer core region will stabilize the outer core against thermal convection.

Figure 1

Mössbauer spectra of $\mathrm{F}{\mathrm{e}}_{0.96}\mathrm{O}$ measured at 1 atm. The residual is shown below the spectrum. The doublets are least-squares fits to the experimental data assuming magnetic and quadrupole split components with different intensities.

Figure 2

Mössbauer spectra of $\mathrm{F}{\mathrm{e}}_{0.96}\mathrm{O}$ measured at room temperature. The residual is shown below in each spectrum. The doublet and sextet are least-squares fits to the experimental data assuming magnetic and quadrupole split components with different intensities.

Figure 3

The pressure dependence of (a) the isomer shift of the nonmagnetic doublet, (b) the quadrupole splitting of the nonmagnetic doublet, (c) the internal magnetic field of the magnetic sextet, and (d) area ratio of the magnetic sextet (solid circles) and the nonmagnetic doublet (gray circles). The isomer shift and quadrupole splitting were calculated from the nonmagnetic component and the internal magnetic field was calculated from the magnetic component.

Figure 4

Phase diagram of FeO. Broken lines represent the phase boundaries in wüstite [3]. Dotted lines in B1 and B8 phase represent the insulator-metallic boundary reported by Ohta et al. [9] and Ozawa et al. [7], respectively. A dashed-dotted line represents $300\mathrm{K}$. Data of this study are shown as squares.