NMR evidence for the 8.5 K phase transition in americium dioxide

Tokunaga, Yo  ; Nishi, Tsuyoshi; Kambe, Shinsaku  ; Nakada, Masami  ; Ito, Akinori; Homma, Yoshiya*; Sakai, Hironori   ; Chudo, Hiroyuki 

We report here the first NMR study of americium dioxide (AmO). More than 30 years ago, a phase transition was suggested to occur in this compound at 8.5 K based on magnetic susceptibility data, while no evidence had been obtained from microscopic measurements. We have prepared a powder sample of AmO containing 90 at.% O and have performed O NMR at temperatures ranging from 1.5 K to 200 K. After a sudden drop of the O NMR signal intensity below 8.5 K, at 1.5 K we have observed an extremely broad spectrum covering a range of 14 kOe in applied field. These data provide the first microscopic evidence for a phase transition as a bulk property in this system. In addition, the O NMR spectrum has been found to split into two peaks in the paramagnetic state, an effect which has not been reported for actinide dioxides studied up to now. We suggest that the splitting is induced by self-radiation damage from the alpha decay of Am.