Neutron-scattering investigation of the electronic ground state of neptunium dioxide
Neutron spectroscopy has been applied to study the electronic ground state of NpO(2) 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 UO(2) to the Np(4+) case. Several mechanisms which could be responsible for the observed splitting are discussed. In the energy range below 15 meV the technique of polarisation analysis has been used together with an incident polarised neutron beam to separate magnetic from vibrational effects. At low temperature, below 25 K, an inelastic line appears (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 Np(4+) 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.
Bibliographic Reference: Article: Journal of Physics : Condensed Matter (1992) No. 4, pp. 3459-3478
Record Number: 199211024 / Last updated on: 1994-12-02
Original language: en
Available languages: en