Powder neutron diffraction and magnetic susceptibility of 248CmO2

Abstract We report the first neutron diffraction experiments on a sample of approximately 55 mg of CmO 2 , using the rare isotope 248 Cm. Preparative techniques have assured that the sample is close to stoichiometry and X-ray patterns show only the expected f.c.c. lines. The neutron diffraction experiments confirm that the sample is nearly stoichiometric and rule out the possibility that the oxygen atoms form a superlattice to accommodate a mixture of Cm 3+ and Cm 4+ ions. Magnetic susceptibility measurements give a μ eff value of (3.36 ± 0.06) μ B for the effective paramagnetic moment, consistent with many other determinations of this property in Cm(IV) compounds, which have been interpreted in the past as indicating substantial Cm 3+ impurity. In view of the contradiction between bulk susceptibility on the one hand and thermogravimetric and diffraction results on the other hand, we suggest that a re-evaluation of the electronic ground states in the actinide dioxides may be needed. Some other contradictory experimental results are given.