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

This is a Mossbauer study of wustite at pressures above 200 GPa using synchrotron Mossbauer spectroscopy. Synthetic Fe 0.96 O-wustite 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 Mossbauer spectrum at 91 GPa consists of both magnetic and nonmagnetic components. The magnetic high-spin component decreases gradually withincreasing 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 above203 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