NMR evidence for the 8.5K phase transition in americium dioxide

We report here the first NMR study of americium dioxide (AmO₂). More than 30 years ago, a phase transition was suggested to occur in this compound at 8.5K based on magnetic susceptibility data, while no evidence had been obtained from microscopic measurements. We have prepared a powder sample of ²⁴³AmO₂ containing 90 at.% ¹⁷O and have performed ¹⁷O NMR at temperatures ranging from 1.5 to 200 K. After a sudden drop of the ¹⁷O NMR signal intensity below 8.5 K, at 1.5K 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 ¹⁷O 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 ²⁴³Am.