NMR studies in actinide dioxides

Tokunaga, Yo  ; Sakai, Hironori   ; Kambe, Shinsaku  

We review our recent NMR studies carried out in dioxides of the uranium and transuranium elements. Neptunium dioxide was thought for many years to be an antiferromagnet; however, neither Mssbauer spectroscopy nor neutron diffraction detected any ordered moment below the transition temperature of 26 K. It is now recognized that the phase transition originates from octupole moments. The octupolar-order scenario was suggested initially by resonant X-ray scattering and then strongly supported by our O NMR studies. Recently, we have also performed an O NMR for the first time on Americium dioxide. Our results include a drastic broadening of the NMR spectrum, after a sudden drop of NMR signal intensity below the transition temperature. These data provide the first microscopic evidence for a phase transition as a bulk property in this system.