Polarized neutron scattering study of the magnetic response across T-N in a single crystal of UO2

We present the results of an inelastic neutron scattering experiment with p olarization analysis performed on a single crystal of UO2. A magnetic inela stic response is observed above T-N in the energy region between 3 and 10 m eV, at least up to 6.5T(N). This result provides evidence of the fact that in the time scale of our experiment the uranium triplet ground state is spl it into three singlets because of a dynamical Jahn-Teller distortion of the oxygen cage, reducing the point symmetry at the uranium site. The position of the peaks and their dispersion are compatible with monoclinic 1-k disto rtions along the three directions of the [100] star. This suggests a pictur e in which as T-N is approached, a correlation builds up between the phases of the corresponding vibrations until, eventually, a static 3-k distortion is obtained below T-N. (C) 1999 American Institute of Physics. [S0021-8979 (99)27208-8].