Researchers have found a new form of iron oxide under pressure

Transition metal oxides, such as iron oxide at extreme conditions, are relevant materials in geosciences, physics and applied materials. Wüstite (Fe1-xO), one of these iron oxides, is thought to be one of the main iron-bearing phases in the lower mantle, when presented in combination with magnesium.

Binary iron oxides, and wüstite in particular, should be straightforward to study as they are chemically very simple. “It has an iron and an oxygen atoms, so on paper it should have no secret to study, but the reality is that under high-pressure conditions, its magnetic, structural and electronic properties are not easy to understand”, explains Ilya Kupenko, ESRF scientist, ERC grantee and corresponding author of the publication.

The situation with wüstite is further complicated by the fact that it is hardly possible to synthesize it as a pure phase with only divalent iron – it has always some amount of trivalent iron impurities. And the study shows that the amount of impurities might additionally influence what phase you end up with.

Mössbauer spectroscopy: A playground for binary sytems

At the ESRF there is a range of techniques available to study iron oxides. In particular, the technique of Mössbauer spectroscopy is indispensable for studies of their electronic and magnetic properties: “It is a bit like a playground for the research on the interplay between structure and magnetism in such simple binary systems”, says Kupenko.

In addition to using Mössbauer Spectroscopy to study the magnetic and electronic properties, the team used beamline ID15B, where they carried out X-ray diffraction analysis to solve the structure of the sample. They also went to PETRA III in Germany. The research highlights how the synergy between spectroscopic and diffraction beamlines allows solving such challenging scientific problems.

Studying single crystals in this kind of compounds at such high pressures has only been possible in the last decade, thanks to the research led by the University of Bayreuth, which has progressively made these studies more and more accessible on different beamlines at the ESRF.

From cubic to rhombohedral to monoclinic

In this publication, the team found that the iron oxide starts as cubic, then it turns into a rhombohedral structure when pressure is applied, and subsequently it transforms into a monoclinic structure when pressure is above 40 GPa. Moreover, it shows that a further distortion to yet another monoclinic phase happens if the samples are heated to high temperatures. This is different from previous theoretical research and observations using powder diffraction, which stated that after the rhombohedral, the samples went to another cubic structure. This research on single crystals proves otherwise.

The new monoclinic structures are similar to each other and to rhombohedral structures observed before, thus, were hard or impossible to identify using powder data. However, the difference in their electronic and magnetic properties is hard to miss. Mössbauer spectra show that two monoclinic phases have a ~ 30 GPa offset for the spin transition pressure which should be related to the different distortions of iron octahedra. There is also a clear difference in the behaviour of the trivalent iron impurities in the two phases. The researchers identified unambiguously the formation of the divalent-trivalent pairs that are magnetically ordered in the rhombohedral and room-temperature monoclinic phases. However, such pairs break down if the sample is transformed to the second monoclinic phase by heating.

The results shed new light on fundamental behaviour of iron oxides. The next step is to study these materials at higher pressures and temperatures. “In high-pressure studies we need a very small beam, and with the new EBS and the new nanoscope on ID14, we can now take this project a step further, reaching pressures of 200 GPa, which would not have been possible before EBS”, explains Xiang Li, the first author of the publication and visiting PhD student at the ESRF.

Reference:

Li, X., et al. Monoclinic distortion and magnetic transitions in FeO under pressure and temperature. Commun Phys 7, 305 (2024). https://doi.org/10.1038/s42005-024-01797-1

Text by Montserrat Capellas Espuny

Top image: The image shows the monoclinic phase (mC4-model) upon compression. Credits: Li, X., et al. Commun Phys 7, 305 (2024).