Like many compounds containing rare earths, EuO has properties that largely depend on the electrons in its tightly bound 4f shell. If there are changes in in the configuration of these electrons, there are usually changes in the material’s magnetism, electrical response and overall structure, too.

For over 50 years, one particular structural change has been noted for EuO: that above a certain high pressure, the material suddenly shrinks in volume. Since then, many theorists have believed that shrinkage is caused by the europium atoms releasing one electron each from their 4f shells, allowing them to pack more tightly. If so, then the oxidation state of europium ought to increase by one – reflecting the extra electronic availability – at the same pressure.

Experiments seemed to confirm this picture, tentatively. At other synchrotrons, X-ray absorption spectroscopy of empty electronic states above EuO’s occupied 4f shell under pressure suggested that the europium’s oxidation state did in fact rise by one. But such studies were indirect probes, and depended on assumptions about how changes in empty electronic states reflect changes in occupied 4f shells. “A satisfactory theoretical framework capable of quantitatively reproducing and explaining [those] experimental spectra has yet to be established,” says materials theorist John Tse at the University of Saskatchewan, Canada.

A few years ago, Tse and others proposed an alternative theory of EuO shrinkage under pressure – that the structural change actually results from a reorganisation of the empty states themselves. At the time it proved controversial, but the latest ESRF results suggest it is the likely explanation.

Using X-ray Raman scattering, a team led by Christoph Sahle, the ID20 scientist in charge, directly tracked the 4f electron shell of EuO as the material was compressed to 66 GPa, and found no evidence for a change in oxidation state during the abrupt volume collapse. Instead, they suggest that the pressure must be reorganising the material’s higher-energy empty electronic states, as Tse and colleagues predicted.

According to Sahle, the experiment relied on a suite of cutting-edge instrumentation, as well as the small beam size of the EBS. “These developments make weak, shallow-core XRS signals under high pressure measurable,” he says. The paper has been highlighted by its publisher, the American Physical Society, as an “editors’ suggestion”.

The researchers believe the growing capabilities of X-ray Raman scattering at the ESRF could now help to resolve similar debates in other rare-earth materials whose behaviour under pressure remains disputed. “The technique employed in our study will be applicable to other systems that involve spin-spin interactions with different numbers of valence electrons,” says Tse.

Reference:

Sahle, Christoph J. et al. Europium valence in pressurized EuO. Physical Review Letters. https://doi.org/10.1103/ktfc-l4yt

Text by Jon Cartwright

Top image: Eu2O3 powder.