Iron takes on a more robust form in order to withstand the stresses of extreme pressures at the Earth’s outer core. The process is known as ‘twinning’.
This is the conclusion of a study by researchers from the SLAC National Accelerator Laboratory who recreated the pressures of the core in the laboratory.
The two lasers they used were aimed at iron samples the size of human hair. First, they created a shockwave that briefly heated and compressed metal.
The second laser — part of SLAC’s Linac Coherent Light Source — enabled the team to probe the effect on the atomic structure of the iron within a billionth of a second.
Iron becomes stronger to withstand extreme stress at its core. This is called “twinning”. The conclusion of a research project by the SLAC National Accelerator Laboratory, which recreated core pressures at the laboratory.
Much of the iron you might encounter out and about — whether for example in buildings, machinery, Victorian lampposts, etc. — has a structure that crystallographers would refer to as ‘body-centred cubic’.
This means that the crystal lattice is arranged into a pattern of nanoscopic cubes, with iron atoms at each cube’s centre, as well as each of the eight corners.
When iron is subjected to higher pressures, however, this structure changes — taking, above 10 gigapascals, a hexagonal form that allows the atoms to pack together more closely.
Arianna Gleason (paper author) and Geologist at SLAC, along with her collaborators, wanted to determine what happens to hexagonal iron if pressure is increased beyond that which was found in the Earth’s core.
‘We didn’t quite make inner core conditions, but we achieved the conditions of the outer core of the planet — which is really remarkable,’ said Professor Gleason.
The team were unsure how iron would respond to such extreme conditions — equivalent to some 360 million times the pressure at the Earth’s surface and as hot as surface of the Sun — as no-one has previously been able to observe such.
Iron undergoes another structural change, similar to the one it experienced at lower pressures when it went from being a cubical structure to becoming a hexagonal.
Professor Gleason stated, “As we push it further, the iron doesn’t know how to deal with this additional stress.”
She continued: ‘It needs to relieve that stress, so it tries to find the most efficient mechanism to do that.’
The coping mechanism that iron resorts to — twinning — sees the arrangement of atoms shunted to the side, rotating all the hexagonal prisms by nearly 90 degrees.
Twinning can be described as a pressure reaction in a variety of minerals and metals such as calcite and quartz.
‘Twinning allows iron to be incredibly strong — stronger than we first thought — before it starts to flow plastically on much longer time scales [than it would have otherwise,’ Professor Gleason said.
‘As we continue to push it, the iron doesn’t know what to do with this extra stress,’ Professor Gleason explained. It must relieve stress and so, it attempts to discover the best way to accomplish that. The coping mechanism that iron resorts to — twinning — sees the arrangement of atoms shunted to the side, rotating the hexagonal prisms by nearly 90°
Professor Gleason stated, “Now, we can give thumbs up or thumbs down to some of the Physics Models for Really Fundamental Deformation Mechanisms.”
“That allows us to improve our predictive capabilities for modeling how extreme materials react under extreme conditions,”
The team also explained that the same techniques could be used to understand the behavior of other materials under severe conditions.
The researchers weren’t sure if the iron they measured would be too fast or too slow to see.
‘The fact that the twinning happens on the time scale that we can measure it as an important result in itself,’ explained paper author and geophysicist Sébastien Merkel of the University of Lille, France.
Twinning can be described as an observable pressure phenomenon in a variety of minerals and metals like calcite. A twinned quartz crystal
Professor Gleason stated that “the future is bright” now that “we’ve developed the way to make those measurements”, and that upgrades to Linac Coherent Light Source have allowed materials to study at higher Xray energies.
This, she explained, will enable studied of ‘thicker alloys and materials that have lower symmetry and more complex X-ray fingerprints’ — while also allowing larger samples to be observed, permitting a more comprehensive look at iron’s behaviour.
Gleason stated, “We’re going get stronger optical lasers with approval to proceed to a new flagship peatawatt laser facility.”
‘That’ll make future work even more exciting because we’ll be able to get to the Earth’s inner core conditions without any problem,’ she concluded.
Full results of this study have been published in Physical Review Letters.