Novel Valence Transition in Elemental Metal Europium around 80 GPa

TL;DR

This study reveals a valence transition in elemental europium around 80 GPa, linked to a structural change and involving complex electron interactions, advancing understanding of its high-pressure properties.

Contribution

The paper provides the first direct evidence of a valence transition in Eu at high pressure, correlating it with structural phase change and electron energy gap behavior.

Findings

Valence transition occurs at ~80 GPa.

Structural transition from monoclinic to orthorhombic phase.

Valence change linked to electron energy gap approaching Coulomb interaction.

Abstract

Valence transition could induce structural, insulator-metal, nonmagnetic-magnetic and superconducting transitions in rare-earth metals and compounds, while the underlying physics remains unclear due to the complex interaction of localized 4f electrons as well as their coupling with itinerant electrons. The valence transition in the elemental metal europium (Eu) still has remained as a matter of debate. Using resonant x-ray emission scattering and x-ray diffraction, we pressurize the states of 4f electrons in Eu and study its valence and structure transitions up to 160 GPa. We provide compelling evidence for a valence transition around 80 GPa, which coincides with a structural transition from a monoclinic (C2/c) to an orthorhombic phase (Pnma). We show that the valence transition occurs when the pressure-dependent energy gap between 4f and 5d electrons approaches the Coulomb interaction. Our discovery is critical for understanding the electrodynamics of Eu, including magnetism and high-pressure superconductivity.