Reentrant valence transition in EuO at high pressures: beyond the bond-valence model

TL;DR

This study investigates the pressure-induced valence changes in EuO, revealing a reentrant Eu valence transition at high pressure that challenges traditional bond-valence models and emphasizes the importance of direct electronic state measurements.

Contribution

The paper provides the first direct experimental evidence of a reentrant Eu valence transition in EuO under high pressure, demonstrating the limitations of bond-valence models in such systems.

Findings

EuO exhibits a 7% volume collapse at ~45 GPa.

Eu valence reenters a lower state after initial increase under pressure.

Bond-valence models fail to accurately predict Eu valence behavior under high pressure.

Abstract

The pressure-dependent relation between Eu valence and lattice structure in model compound EuO is studied with synchrotron-based x-ray spectroscopic and diffraction techniques. Contrary to expectation, a 7% volume collapse at $\approx$ 45 GPa is accompanied by a reentrant Eu valence transition into a $\emph{lower}$ valence state. In addition to highlighting the need for probing both structure and electronic states directly when valence information is sought in mixed-valent systems, the results also show that widely used bond-valence methods fail to quantitatively describe the complex electronic valence behavior of EuO under pressure.