Lattice distortion and quadrupolar ordering in UO2

The origin of the monoclinic lattice distortion observed in the low-temperature antiferromagnetic phase of U${\mathrm{O}}_2$ is studied. Taking into account the coupling between the internal and external strains, characteristic of the fluorite structure, and using the experimental phonon data, it is shown that the Jahn-Teller mechanism alone strongly favors a uniform distortion with ferroquadrupolar order of the uranium ions. This is in contrast to the observed monoclinic cell-doubling deformation. It is demonstrated that the direct electrostatic quadrupole-quadrupole interactions, on the other hand, favor the antiferroquadrupolar order. Hence, the observed structure is due to a combined effect of the Jahn-Teller and direct electrostatic interactions.