17O NMR Studies of Uranium Oxides

Abstract

The 17O NMR of 17O-enriched UO2, BiUO4, and KUO3 has been observed. A paramagnetic NMR shift was observed for UO2 at 192.5, 299.5, and 405.2°K. The average isotropic coupling constant, As, calculated from these shifts is 2.84 × 10−5 cm−1. In this calculation of As the magnitude of 〈sz〉 on the uranium atom in UO2 is determined assuming a 3H4 ground state for the U4+. Paramagnetic shifts were also observed for BiUO4 and KUO3 at 299.5°K. The value of As for BiUO4 was determined to be 3.79 × 10−4 cm−1. This value was calculated after assuming a 2F5/2 ground state for U5+. The same assumption in the case of KUO3 yielded a value of 1.08 × 10−3 cm−1 for As. The validity of the assumption of pure Russell–Saunders ground states for these compounds is discussed. For KUO3 the previously published spin–orbit coupling constant and crystal field terms are used to calculate a more reliable value of 〈Sz〉 on the central uranium atom. These calculations lead to a value of 7.01 × 10−4 cm−1 for As. The important conclusions of this study are that the fractional s character in these compounds is small and that there is a negative spin density on the 17O nuclei. Apparently the major contribution to the hyperfine interaction arises from the involvement of excited metal orbitals and/or from exchange polarization of core electrons. The NMR spectra of KUO3 and BiUO4 show the effect of anisotropic hyperfine interactions. The anisotropic interactions in BiUO4 are shown to be predominantly π in character. An analysis of the KUO3 spectrum indicates that anisotropic interactions are predominantly π in character and that Aπ ∼ − 6 × 10−5cm−1. The linewidth of the isotropic UO2 resonance is interpreted in terms of the Moriya spin-exchange mechanism and the value of ωe is found to be 1.2 × 10+13 sec−1.