Direct observation of electric-quadrupolar order in UO$_{2}$

TL;DR

This paper provides the first direct experimental evidence of long-range electric-quadrupolar antiferro order in UO₂ using resonant x-ray scattering, clarifying the nature of quadrupolar ordering in uranium dioxide.

Contribution

It presents the first direct experimental observation of electric-quadrupolar order in UO₂ and proposes a model for the phase diagram of UO₂-NpO₂ solid solutions.

Findings

Resonant x-ray scattering confirms quadrupolar order in UO₂.

Quadrupolar order influences the phase diagram of UO₂-NpO₂.

Frustration and short-range order occur in certain composition ranges.

Abstract

We report direct experimental evidence for long-range antiferro ordering of the electric-quadrupole moments on the U ions. Resonant x-ray scattering experiments at the uranium $M_{4}$ absorption edge show a characteristic dependence in the integrated intensity upon rotation of the crystal around the scattering vector. Although quadrupolar order in uranium dioxide was advocated already in the 1960s no experimental evidence for this phenomenon was provided until now. We conclude with a possible model to explain the phase diagram of the solid solutions of UO$_{2}$ and NpO$_{2}$. We suggest that in the region $0.30 < x < 0.75$ neither the transverse nor the longitudinal quadrupole ordering can dominate, leading to frustration and only short-range ordering.