Orbital Ordering in Actinide Oxides: New Perspectives on Old Problems

The behaviour of the actinide oxides has been of major interest since the 1940s. UO2 is known to exhibit antiferromagnetism since neutron experiments of the 1960s. However, the puzzle of the lack of ordered magnetism in NpO2 (also dating from the same period) was recently solved by the discovery [1, 2] that the ordering phenomenon in NpO2 is not connected with magnetic dipole moments (as in UO2), but rather with the long-range ordering (at 25 K) of octupole magnetic moments which then induces ordering of the anisotropic 5f charge distribution surrounding the Np4+ ion. Such ordering of the 5f quadrupole charge distribution cannot be observed with neutrons (or muons) but can be measured by resonant x-ray scattering. Following the work on pure NpO2, we have re-examined single crystals of (U0.75Np0.25)O2 and shown that both the U and Np ions in this material exhibit dipole and quadrupole ordering with transverse symmetry [3]. To a lesser or greater extent all theoretical models of UO2 emphasize the importance of the interplay between the Jahn- Teller and quadrupolar interactions. Allen [4] indeed proposed that the quadrupoles ordered and the subsequent internal strain would lead to a change in the position of the oxygen atoms without giving rise to an external change in the symmetry of UO2. Although the internal distortion of the oxygen cage was observed by neutron diffraction [5], no direct evidence for the ordering of electric quadrupole moments below TN in UO2 was obtained.

WILKINS S.B.;  PAIXAO J.A.;  CACIUFFO R.A.;  DETLEFS C.;  REBIZANT Jean;  LANDER Gerard; 

2005-10-11

Royal Society of Chemistry

JRC30435

https://publications.jrc.ec.europa.eu/repository/handle/JRC30435,