Solid-State NMR Study on Actinide Dioxides
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Solid-State NMR Study on Actinide Dioxides Yo Tokunaga1 Hironori Sakai1, Shinsaku Kambe1, Hiroyuki Chudo1, Masahiko Osaka2, Shuhei Miwa2, Tsuyoshi Nishi3, Masami Nakada3, Akinori Itoh3 and Yoshiya Homma4 1
Advanced Science Research Center, Japan Atomic Energy Agency, 2-4 Shirakata-Shirane, Tokai, Naka, Ibaraki, 319-1195, Japan. 2 Oarai Research and Development Center, Japan Atomic Energy Agency, Oarai, Higashi-Ibaraki, Ibaraki 311-1393, Japan 3 Nuclear Science and Engineering Directorate, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan. 4 Institute for Materials Research, Tohoku University, Oarai, Higashi-Ibaraki, Ibaraki 311-1313, Japan. ABSTRACT 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 Amcontaining dioxide systems, (Pu1-xAmx)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 Peff=1.38 μB. The value suggests the 5f 5 (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. INTRODUCTION Actinide dioxides (AnO2 : An=U, Np, Pu, Am) represent possibly the most intensely studied series of any actinide compounds. From chemical and industrial perspectives, this interest has mostly stemmed from their use as nuclear fuels. Recently, however, AnO2 has attracted a great deal of attention in the research field of f-electron physics. In Table 1, we summarize the crystallographic and physical properties of AnO2. They are all cubic insulators with rather well-localized 5f electrons. The actinide ions have the same tetravalent state, so that the number of 5f electrons per actinide ion varies systematically, with two for U4+, three for Np4+, four for Pu4+ and five for Am4+ in the same cubic fluorite structure. The variation of the 5f electrons number results in the variation of their crystalline electric field (CEF) ground state. In f-electron systems, the localized f-electron state is represented in terms of multipole degrees of freedom. Available multipoles on each material are dependent on the character of the CEF ground state. In general, with a low crystal symmetry, higher order multipoles ( such as quadrupole, octupole, hexadecapole, etc. ) are generally quenched. In the cubic symmetry of
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AnO2, on the other hand, even the higher order multipoles are not quenched, and affect the physical properties at low temperatures. If th
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