17O NMR Studies of Uranium Oxides

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, $A_s$ , calculated from these shifts is 2.84 × 10⁻⁵ cm⁻¹. In this calculation of $A_s$ the magnitude of ${\mathrm{〈S}}_z〉$ on the uranium atom in UO₂ is determined assuming a ${}^3{H}_4$ ground state for the U⁴⁺. Paramagnetic shifts were also observed for BiUO₄ and KUO₃ at 299.5°K. The value of $A_s$ for BiUO₄ was determined to be 3.79 × 10⁻⁴ cm⁻¹. This value was calculated after assuming a ${}^2{F}_{\text{5/2}}$ ground state for U⁵⁺. The same assumption in the case of KUO₃ yielded a value of 1.08 × 10⁻³ cm⁻¹ for $A_s$ . 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 ${\mathrm{〈S}}_z〉$ on the central uranium atom. These calculations lead to a value of 7.01 × 10⁻⁴ cm⁻¹ for $A_s$ . 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 $\pi$ in character. An analysis of the KUO₃ spectrum indicates that anisotropic interactions are predominantly $\pi$ in character and that $A_{\pi }{\text{\hspace{0.17em}∼\hspace{0.17em}−\hspace{0.17em}6\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−5}}{\mathrm{cm}}^{\text{−1}}$ . The linewidth of the isotropic UO₂ resonance is interpreted in terms of the Moriya spin‐exchange mechanism and the value of ${\omega }_e$ is found to be 1.2 × 10⁺¹³ sec⁻¹.