Exploring the key states of iron with synchrotron tools

Iron (Fe) is a signature constituent of the Earth. It is prevalent in the planet’s core, which generates Earth’s protective magnetic field. Iron is also allotropic, having different structures and electromagnetic properties as temperature and pressure are varied. However, serious questions remain about iron’s phase changes with pressure, in particular for the transition from the α-Fe form at ambient conditions to the to ϵ-Fe form, the main constituent of the Earth’s core. Recently, two physicists employed synchrotron X-ray diffraction (XRD) to test current theories for α-Fe/ϵ-Fe structures and transitions. They report their findings in Applied Physics Letters.

Agnès Dewaele and Gaston Garbarino conducted the experiment at the European Synchrotron Radiation Facility (ESRF) in Grenoble. Dewaele says it was important to conduct their research at very low temperature in order to effectively explore α-Fe’s and ϵ-Fe’s electronic and magnetic properties and establish an equation of state for ϵ-Fe with the electrons in the ground state. The researchers placed the iron samples in diamond anvil cells surrounded by helium, which served as the pressure-transmitting medium. This state-of-the-art pressure methodology provided excellent conditions for studying the two iron structures with angular-dispersive XRD, according to Dewaele.

Under comparable temperature and pressure conditions, theory and early experiments indicated that an electronic topological phase transition (ETT) occurred during the transformation from α-Fe to ϵ-Fe. An ETT for iron would be evidenced by changes in the ratio of two lattice parameters and in the electronic properties. However, under their rigorous conditions, Dewaele and Garbarino found no such evidence for an ETT. Dewaele says this is a main contribution of the experiment. She adds that her and Garbarino’s investigation offers researchers a baseline reference for the behavior of iron at low temperature and under high pressure.

Source: “Low temperature equation of state of iron,” by Agnès Dewaele and Gaston Garbarino, Applied Physics Letters (2017). The article can be accessed at https://doi.org/10.1063/1.4989688 .