Metal undergoes novel transition under extreme pressure

Iron oxide was subjected to conditions similar to those at the depth where the Earth's innermost two layers meet.

At 1,650C and 690,000 times sea-level pressure, the metal changed the degree to which it conducted electricity.

But, as the team outlined in Physical Review Letters, the metal's structure was surprisingly unchanged.

The finding could have implications for our as-yet incomplete understanding of how the Earth's interior gives rise to the planet's magnetic field.

While many transitions are known in materials as they undergo nature's extraordinary pressures and temperatures, such changes in fundamental properties are most often accompanied by a change in structure.

These can be the ways that atoms are arranged in a crystal pattern, or even in the arrangement of subatomic particles that surround atomic nuclei.

A team at the Carnegie Institution for Science subjected the material to pressures up to 1.4 million times atmospheric pressure at sea level, and temperatures up to 2,200C.

They found that it pulls off the trick of changing its electrical properties without any shifting of shape - it can be an insulator or conductor depending just on temperature and pressure.

Combined with computer simulations of just what was going on with the material's electrons, the group claim that the results show a new type of metallisation.

"At high temperatures, the atoms in iron oxide crystals are arranged with the same structure as common table salt," said Ronald Cohen, a co-author of the study. "Just like table salt, iron oxide at ambient conditions is a good insulator—it does not conduct electricity."

"Our new results show, instead, that iron oxide metallises 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."

A mixture of magnesium and iron oxide makes up much of the Earth's mantle - the solid layer just outside the planet's liquid outer core. The fact that iron oxide behaves as a metal means it will electrically link the core and mantle, affecting the way the magnetic field makes its way to the Earth's surface and beyond.