Competing lattice modes and the quadrupolar ordering in UO2

Below Tsub(N) = 30.8 K UO2 is in an antiferromagnetic state and an internal lattice distortion of the cell-doubling M5-type is observed. Theory based on the Jahn-Teller effect predicts another type of distortion. We include the dynamical coupling of internal and external strains to show that the observed distortion is energetically even more unfavorable than previously thought, unless interactions other than of the Jahn-Teller type are taken into account. It is shown that the direct quadrupole-quadrupole interaction enhanced by antishielding may lead to the observed structure. A diffraction experiment measuring the coupling between the internal and external strains in the compound is proposed. (orig.)