NMR Study of Na3UF8 Structure and Hyperfine Effects

Nuclear magnetic resonance spectra of polycrystalline Na₃UF₈ were studied experimentally. The magnetic field dependence of the ¹⁹F second moment allowed a separation of its field‐dependent and field‐independent parts. The temperature dependence of the field‐independent part exhibits no discontinuities attributable to phase transitions and only a slow transition at about 250°K attributed to hindered molecular rotation of the isolated UF₈ units. The temperature dependence of the field‐dependent part also exhibits no discontinuities. It does reflect the paramagnetic susceptibility which obeys, to within the accuracy of the experiment, the Curie law. The isotropic chemical shift is small, i.e., − 0.07% ± 0.01% with respect to F₂ at 292°K and − 0.11% ± 0.01% at 175°K. The even smaller chemical‐shift anisotropy is approximately 25% of the isotropic shift. These shift data are consistent with a system containing one $\text{5f}$ electron as the unpaired spin. The ¹⁹F spectra and the temperature dependence of the field‐dependent second moment require the existence of inequivalent ¹⁹F nuclei. This is not consistent with the published crystal structure of Na₃UF₈ characterized as $I_4\mathrm{\hspace{0.17em}/\hspace{0.17em}mmm}$ .