Direct observation of electric-quadrupolar order in UO2

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 UO2 and NpO2. We suggest that in the region 0.30 < x < 0.75 of U(1-x)NpxO2 neither the transverse nor the longitudinal quadrupole ordering can dominate, leading to frustration and only short-range ordering.