New Kind of Metal in the Earth
Washington, D.C.,16 December 2011- The crushing pressures and intense temperatures in Earth’s deep interior squeeze atoms and electrons so closely together that they interact very differently. With depth materials change. New experiments and supercomputer computations discovered that iron oxide undergoes a new kind of transition under deep Earth conditions. Iron oxide, FeO, is a component of the second most abundant mineral at Earth’s lower mantle, ferropericlase. The finding, published in an upcoming issue of Physical Review Letters, could alter our understanding of deep Earth dynamics and the behavior of the protective magnetic field, which shields our planet from harmful cosmic rays.
Ferropericlase contains both magnesium and iron oxide. To imitate the extreme conditions in the lab, the team including coauthor Ronald Cohen of Carnegie’s Geophysical Laboratory, studied the electrical conductivity of iron oxide to pressures and temperatures up to 1.4 million times atmospheric pressure and 4000°F—on par with conditions at the core-mantle boundary. They also used a new computational method that uses only fundamental physics to model the complex many-body interactions among electrons. The theory and experiments both predict a new kind of metallization in FeO.
Compounds typically undergo structural, chemical, electronic, and other changes under these extremes. Contrary to previous thought, the iron oxide went from an insulating (non-electrical conducting) state to become a highly conducting metal at 690,000 atmospheres and 3000°F, but without a change to its structure. Previous studies had assumed that metallization in FeO was associated with a change in its crystal structure. This result means that iron oxide can be both an insulator and a metal depending on temperature and pressure conditions.
“At high temperatures, the atoms in iron oxide crystals are arranged with the same structure as common table salt, NaCl,” explained Cohen. “Just like table salt, FeO at ambient conditions is a good insulator—it does not conduct electricity. Older measurements showed metallization in FeO at high pressures and temperatures, but it was thought that a new crystal structure formed. Our new results show, instead, that FeO metallizes without any change in structure and that combined temperature and pressure are required. Furthermore, our theory shows that the way the electrons behave to make it metallic is different from other materials that become metallic.”
“The results imply that iron oxide is conducting in the whole range of its stability in Earth’s lower mantle.” Cohen continues, “The metallic phase will enhance the electromagnetic interaction between the liquid core and lower mantle. This has implications for Earth’s magnetic field, which is generated in the outer core. It will change the way the magnetic field is propagated to Earth’s surface, because it provides magnetomechanical coupling between the Earth’s mantle and core.”
“The fact that one mineral has properties that differ so completely—depending on its composition and where it is within the Earth—is a major discovery,” concluded Geophysical Laboratory director Russell Hemley.
FeO is the simplest iron oxide, and thus since most minerals in the Earth contain iron, the prototype for all iron bearing and transition metal bearing minerals.
Using the standard model (called band theory, or density functional theory (DFT)) for understanding materials, FeO would be a metal at ordinary pressure and temperature conditions, but it is a good insulator. It does not conduct electricity. It is known as a prototypical Mott or charge-transfer insulator, which means that it is the correlations between the electrons that make it an insulator. In other words, if the electrons could run about nearly freely, it would be a metal, but the motions of the electrons are correlated, and this essentially localizes them so that each electron tends to stay with a given iron atom for a long time, thus not allowing electrical current to flow. The theoretical calculations of Cohen and Haule showed that at high temperatures and pressures this correlated state breaks down and FeO reverts to behaving more like the standard model.
FeO is the simplest iron oxide, and thus since most minerals in the Earth contain iron, the prototype for all iron bearing and transition metal bearing minerals. Twenty-five years ago, Knittle and Jeanloz observed metallization in FeO under shock conditions. 1 They assumed it was a structural transition to the B2 or B8 structures, and Fei and Mao (1994) suggested that they had seen the B8 (NiAs) structure based on diamond anvil measurements, 2 but recently it was shown that the B8 phase does not form at those high temperatures, but at lower temperatures and high pressures. 3 So the old measurements were a mystery, but it is important for geophysics to know if any major oxides or silicates metallize within Earth’s mantle. Ohta, Cohen and colleagues applied in situ, high temperature resistivity and X-ray studies in the diamond anvil cell, and state-of the-art self-consistent many-body theory to study FeO under high pressures and temperatures. They found a new kind of isostructural insulator to metal transition that agrees with the shock experiments, which has important implications for metallization in transition metal compounds and to geophysics. For example, metallization in the FeO endmember implies two (Mg,Fe)O magnesiowüstite phases in the Earth, rather than one, one being insulating and the other metallic. The proportions of the two phases would depend on the amount of iron and temperature at a given pressure. Magnesiowüstite is believed the second most common mineral in the deep Earth, so this has implications for electrical and thermal conductivity in the Earth, important for modeling Earth’s magnetic field and dynamics.
1 Knittle, E. & Jeanloz, R. High-pressure metallization of FeO and implications for the earth's core. Geophys. Res. Lett. 13, 1541-1544 (1986).2 Fei, Y. & Mao, H.-K. In situ determination of the NiAs phase of FeO at high pressure and temperature. Science 266, 1668-1680 (1994).3 Ozawa, H., Hirose, K., Tateno, S., Sata, N. & Ohishi, Y. Phase transition boundary between B1 and B8 structures of FeO up to 210 GPa. Phys. Earth Planet. Int. 179, 157-163 (2010).
This study has been accepted for publication by the journal Physical Review Letters. A preprint is available at the arxiv.