Solid-State NMR Study on Actinide Dioxides

Besides the importance of the actinide dioxide series as a nuclear fuel, the magnetic properties of these compounds at low temperatures are particularly interesting. Their surprisingly varied physical properties at low temperatures stimulate continuing interest for both theory and experiment. Recently, we have performed ¹⁷O-NMR studies for the first time on Pu and Am containing dioxide systems, (Pu_1-xAm_x)O₂. For the x=0.09 sample, a temperature-dependent NMR line broadening has been observed at low temperatures. By comparing the experimental data with the results of NMR line simulations, we have estimated the effective moment of Am ions to be P_eff=1.38 μB. The value suggests the 5f⁵ (Am⁴⁺) state of the Am ion in PuO₂. For the x=1 (=AmO₂) sample, on the other hand, our ¹⁷O-NMR data provide the first microscopic evidence for a phase transition at 8.5 K as a bulk property in this system. A spectrum with a triangular line shape indicates that the internal field is distributed very nearly randomly in the ordered state. (author)