Magnetic moment distribution in uranium dioxide

The magnetic form factor of uranium in UO2 was examined by means of neutron diffraction. Uranium dioxide (CaF2 structure) orders antiferromagnetically at 30.80K with the type I magnetic structure. The intensities of 1250 magnetic Bragg reflections (reducing to 71 independent hkl values and giving full three-dimensional data for sin theta/lambda less than or equal to 0.83 A-1) were measured with a well-characterized stoichiometric single crystal at 50K. The values of (μf) plotted versus sin theta/lambda show considerable asphericity for sin theta/lambda greater than 0.5 A-1. Comparisons with free-ion (+ crystal field) calculations give an experimental magnetic moment of 1.78 (4) μ/sub B/ and indicate that the general shape of the form factor is in agreement with theory for low scattering angles, but that the nonspherical contributions at higher angles are at least a factor of five greater than expected. The significance of these results is discussed in terms of the extended nature of the 5f wavefunctions in UO2. (auth)