Evidence of a lattice distortion in NpO2 below 25K from neutron magnetic inelastic scattering

Modifications of the crystal field ground state of NpO2 below the phase transition at 25 K have been investigated by magnetic neutron spectroscopy. The results show a splitting in the low temperature phase of the cubic Γ8(2) quartet ground state into two doublets separated by 7.2 meV. This supports the hypothesis that the phase transition involves the quadrupolar ordering of the Np4+ ions by a collective Jahn-Teller distortion of the oxygen sublattice. The observed amplitude of the splitting is consistent with an oxygen displacement of the order of 0.02 A, which is below the present limits of resolution of neutron diffraction experiments. (author)