Journey to the centre of the Earth
Studying how iron behaves at the extreme conditions inside the earth – at pressures of around 300 GPa and where temperatures are estimated to be nearly 6000°C – can help scientists to understand the structure and geodynamics of our planet.
How does iron affect our planet?
Iron’s structure and behaviour can change with the extreme conditions inside the Earth. Variations in pressure or temperature can cause changes in the electronic configuration of iron contained in minerals. This can affect the iron-containing mineral’s physical properties, such as its electrical conductivity, magnetism or heat flow. Iron’s behaviour can therefore have a large-scale effect on the physical and geochemical properties of Earth’s rocks, melts, and fluids, affect the transport of heat through the planet, cause volcanic ‘hotspots’ and impact on the global hydrogen and water cycles.
Hot on the trail of iron’s melting point
For over 20 years, scientists have been trying to determine the precise melting temperature of pure iron under conditions at the centre of the Earth in order to define the boundary between the Earth’s liquid iron outer core and its lower mantle, nearly 3000 km below the ground. One of their biggest challenges is to avoid contaminating iron with other elements, such as carbon or sulphur, which can lower its melting temperature dramatically. Another is to precisely determine the pressure and temperatures that prevail in the small heating hot-spot of the sample (around 5-10 µm).
An international team of researchers from the ESRF, Sorbonne University in Paris, Diamond Light Source in the UK and the Geodynamics Research Centre in Japan have recently conducted experiments to tackle both of these challenges. The scientists placed a 5-µm-thick foil of pure iron into a laser-heated diamond anvil cell at ESRF beamline ID24. They gradually subjected it to the pressures found at the boundary between the Earth’s core and mantle – around 130 GPa – and heated it using two lasers focused on the sample surface.
Changes in the atomic and electronic structure, identified with a technique called X-ray absorption near edge structure (XANES) spectroscopy, indicated the phase transition from solid to liquid iron. Recovered samples were then carefully checked for contamination using X-ray diffraction techniques on beamline ID27, and with electron microscopy to verify the X-ray observations. With this multi-approach method, the scientists determined iron’s melting temperature to 4250 ± 250 K (3977 +/- 23°C) at a core-mantle boundary pressure of 136 GPa.
The results of the study, which was published in the journal Geophysical Research Letters, have important implications for determining what other elements may be present in the Earth's core. Seismic data suggest the presence of nickel and cobalt as well as light elements such as oxygen, sulphur and carbon but researchers don't yet know the quantities of these elements in the Earth's core.
"The precisely determined melting curve of pure iron is essential in answering this question. Researchers are now investigating how iron's melting temperature is affected by the addition of other elements in order to better determine what the Earth’s core is made of,” says Angelika Rosa (left), ID24 beamline scientist and co-author of the paper.
Such research will be bolstered by the ESRF’s new Extremely Brilliant Source (EBS). The smaller beam size and substantial increase in flux means that researchers will be able to probe very small samples at even further extremes of pressure and temperature, going ever deeper in their mission to shine a light on the properties of materials such as iron at the centre of the Earth.
Keep up with the latest research and news from the ESRF's Matter at Extremes group on Twitter at: @MEx_ESRF
Iron for industry
But it’s not just at the centre of our planet that iron plays a defining role. Three-thousand kilometres above the Earth’s core, it's abundance in rocks as exploitable iron ores and the ease in which it can be worked has meant that humans have prized iron from long before the Iron Age to well after the Industrial Revolution.
Rust to railways: iron's industrial uses
Iron oxides have been used as pigments since prehistoric times and a dagger containing meteoritic iron was famously discovered in Tutankhamun’s tomb from 1323 BC. Early humans quickly learned that adding a small amount of carbon to relatively soft iron resulted in the super-strength alloy steel, and steel weapons have been found from as early as 1800 BC. Iron production expanded and improved across Europe during the Industrial Revolution in the 1760s to the advent of the railways in the 1830s.
Today, iron is the most widely used of the metals, with steel used in infrastructure and machinery, iron oxides used in magnetic storage devices for computers, and iron catalysts utilised in industrial processes such as the production of ammonia and hydrocarbons for fuels and industrial lubricants.
But with innovative research into the intriguing properties of this element continuing apace, what part might iron play in tomorrow’s technology?
Forging the way for futuristic new technology
Scientists are synthesising novel forms of iron oxides under extreme pressures and temperatures in order to explore the exciting new properties they may reveal. They are interested in a phenomenon called charge ordering that can lead to pronounced and often unexpected changes in electronic band structure, thereby affecting the physical and chemical properties of a material. For example, the charge ordering transition in magnetite (Fe3O4) changes it from an electrical conductor to an insulator.
In a recent study published in Nature Communications, scientists from Germany, Russia, India, Denmark and the ESRF studied single crystals of the novel iron oxide Fe4O5 to investigate the effect of high pressure on the low-temperature charge-ordering processes in this material. They conducted a series of experiments on Fe4O5 in a diamond anvil cell mounted in a cryostat, in which they varied the temperature to below 120 K (-150°C) and the pressure up to 25 GPa. At various conditions, they collected data on different sample properties using a range of techniques such as Mössbauer spectroscopy at ID18 (left) to analyse the electronic structure and single crystal X-ray diffraction at ID27 to observe the crystal structure.
The researchers found that cold compression of the material stimulated a transfer of electrons between different iron atoms, leading to the formation of novel charge-ordered phases. Thus, they could effectively 'tune' the charge ordering pattern and charge-ordering temperature in Fe4O5 by varying the amount of applied pressure.
“Controlling the charge-ordering processes in iron oxide materials opens a portal to capabilities that could eventually point the way to fabrication of atomic-scale switches for the next generation of nanoelectronic devices, for example memory elements with ultrahigh density,” says Sergey Ovsyannikov, lead author of the paper.