NMR Study on AmO₂: Comparison with UO₂and NpO₂

Abstract

Recently, 17 O NMR has been performed for the first time on AmO 2 . We have observed a drastic broadening of the NMR spectrum, along with a sudden drop of NMR signal intensity, below T 0 = 8.5 K. These data provide the first microscopic evidence for a phase transition at this temperature as a bulk property of this system. In this paper, we compare NMR results for AmO 2 with those from UO 2 and NpO 2 . In the ordered state we have recorded an 17 O NMR spectrum with a triangular line shape, which resembles neither that of UO 2 nor NpO 2 . This spectrum indicates that the internal local field H int is distributed very nearly randomly from zero to a maximum value, that is, H max int ∼ 7kOe. The temperature dependences of the Knight shift and the nuclear relaxation rate are also compared.KEYWORDS: actinide dioxide, NMR, phase transition, self-radiation effect Actinide dioxides (AnO 2 : An = U, Np, Pu and Am, etc.) are all cubic insulators with rather well-localized 5 f electrons. The actinide ions have the same tetravalent state, so that the number of 5 f electrons per actinide ion varies systematically, with two for U 4+ , three for Np 4+ , four for Pu 4+ and five for Am 4+ in the same cubic fluorite structure. In f -electron systems, the f -electron state is represented in terms of multipole degrees of freedom. Available multipoles on each material are dependent on the character of the crystalline electric field (CEF) ground state determined by the number of f -electrons as well as the symmetry of the crystal. In the cubic symmetry of AnO 2 , even the higher order multipoles are not quenched, and are found to affect the physical properties at low temperatures.UO 2 exhibits an antiferromagnetic (AFM) phase transition at T N = 30.8 K. The magnetic order has the transverse triple-q structure with an ordered moment of 1.74µ B /U atom.1-5) Recently, resonant X-ray scattering measurements have revealed the occurrence of long-range antiferroquadrupolar (AFQ) ordering as a secondary order parameter below T N . 6) NpO 2 is known to exhibit a rather mysterious phase transition at T 0 = 26 K. The compound was thought for many years to be an antiferromagnet; 7-10) however, neither Mössbauer spectroscopy 11,12) nor neutron diffraction [13][14][15] detected any ordered moment below T 0 . It is now recognized that the phase transition originates from an octupole moment of Γ 5 symmetry (or other high-order multipoles with the same symmetry) allowed by the Γ 8 quartet CEF ground state of the Np 5f electrons. [16][17][18][19][20][21][22][23][24][25] The longitudinal triple-q Γ 5 antiferrooctupolar (AFO) order scenario was suggested initially by resonant X-ray scattering 17) and then strongly supported from a microscopic viewpoint by 17 O NMR studies. 20,21,[26][27][28] AmO 2 also exhibits a phase transition at T 0 =8.5 K. However, owing to the limited number of experimental data, the mechanism of this phase transition has remained a mystery for more than 30 years. The phase transition was first reported from magnetic susceptibility ...

High-Resolution Solid-State Oxygen-17 NMR of Actinide-Bearing Compounds: An Insight into the 5f Chemistry

“…similar studies on the rare-earth-bearing compounds). The shifts measured here were compared with those reported in static conditions by Tokunaga et al ,− and Eastman et al For UO 2 , NpO 2 , and PuO 2 , the results are quite similar. For AmO 2 , the difference in shift is probably caused by the differences in the samples themselves (e.g., stoichiometry and irradiation-induced damage) and also by the heating induced by the sample rotation.…”

Microscopic theory of the insulating electronic ground states of the actinide dioxides AnO2(An = U, Np, Pu, Am, and Cm)

“…101 Notably, there are many existing experimental challenges to distinguish the essential bulk contribution of AmO 2 due to the strong self-radiation damage caused by alpha decay. [102][103][104][105] For the next actinide dioxide CmO 2 , only a few experiments have thus far been reported for the detailed constitution of its ground state. 19,106 Cm 4+ has six electrons in the 5f shell, producing a nonmagnetic 7 F 0 ground state from Hund's rules.…”

Crystal structure and magnetism of actinide oxides: a review

“…Similar to NpO 2 , Mössbauer spectroscopy [33] and neutron diffraction measurements failed to detect any magnetic order below T 0 = 8.5 K in AmO 2 [34]. However, NMR measurements on AmO 2 performed by Tokunaga et al [14,15] confirmed the phase transition seen at T 0 , although the NMR spectrum of AmO 2 is rather different from that seen in the AFM ground state of UO 2 [217] or the multipolar ground state of NpO 2 [218].…”

“…Over the course of the experiment, significant NMR intensity change occurred, indicating the electronic ground state is extremely sensitive to disorder induced by selfirradiation through the decay of Am [14]. Moreover, the NMR spectrum of AmO 2 is rather different from that seen in the AFM ground state of UO 2 or the multipolar ground state of NpO 2 [218]. The intense radioactivity and self-induced disorder present in any AmO 2 powder sample make experimentation particularly delicate and, thus, limit the available experimental techniques.…”