NEUTRON-SCATTERING INVESTIGATION OF THE ELECTRONIC GROUND-STATE OF NEPTUNIUM DIOXIDE

Neutron spectroscopy has been applied to study the electronic ground state of NpO2 both in the paramagnetic and in the ordered phase, below T(c) = 25 K. The magnetic inelastic scattering cross section shows a broad peak which is split into two components and is centred at about 55 meV. This inelastic peak is shown to originate from excitations between the GAMMA(8)(2) and GAMMA(8)(1) crystal field quartets. Its position is in agreement with the value estimated by scaling the crystal field potential of UO2 to the Np4+ case. Several mechanisms which could be responsible for the observed splitting are discussed. In the energy range below 15 meV the technique of polarization analysis has been used together with an incident polarized neutron beam to unambiguously separate magnetic from vibronic effects. At low temperature, below 25 K we see the appearance of an inelastic line (at an energy transfer of 6.4 meV) that indicates a lifting of the degeneracy of the GAMMA(8) ground state. 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 angstrom, which is below the present limits of resolution of neutron diffraction experiments.