The gamma->alpha phase transition in cerium is not isostructural

TL;DR

This study reveals that the gamma to alpha phase transition in cerium involves a symmetry change driven by valence electrons, evidenced by a quadrupole charge density that breaks spatial symmetry, despite unchanged atomic positions.

Contribution

It provides the first evidence of symmetry lowering driven solely by valence electron degrees of freedom during the cerium phase transition.

Findings

Detection of quadrupole electron charge density in alpha-Ce

Symmetry lowering at gamma->alpha transition confirmed

Valence electrons drive the phase transition without atomic displacement

Abstract

Using a small amount of radioactive 111Cd atoms implanted in cerium lattice to probe the hyperfine electric quadrupole interaction, we have detected a quadrupole electron charge density component in alpha-Ce, which is absent in gamma-Ce. The appearance of the quadrupole density in alpha-Ce predicted by the quadrupole scenario of the phase transition demonstrates that the spacial symmetry is lowered at the gamma->alpha phase transition. This finding makes cerium the first element where the symmetry change is driven exclusively by the valence electron degrees of freedom while the atomic centers of mass (cerium nuclei) occupy the face centered cubic positions. We also report on nuclear quadrupole interactions in other high pressure phases of cerium: alpha' (C2/m space symmetry) and alpha' (alpha-U structure).