Probing the ${\mathrm{Pu}}^{4+}$ magnetic moment in ${\mathrm{PuF}}_4$ with ${}^{19}\mathrm{F}$ NMR spectroscopy

The magnetic fields produced by ${\mathrm{Pu}}^{4+}$ centers have been measured by ${}^{19}\mathrm{F}$ NMR spectroscopy to elucidate the Pu-F electronic interactions in polycrystalline ${\mathrm{PuF}}_4$. Spectra acquired at applied fields of 2.35 and 7.05 T reveal a linear scaling of the ${}^{19}\mathrm{F}$ line shape. A model is presented that treats the line broadening and shifts as due to dipolar fields produced by Pu valence electrons in localized noninteracting orbitals. Alternative explanations for the observed line shape involving covalent Pu-F bonding, superexchange interactions, and electronic configurations with enhanced magnetic moments are considered.

Figure 1

Monoclinic crystal structure of ${\mathrm{PuF}}_4$ illustrating the coordination of Pu (blue) with eight fluorine atoms (green) and the near linearity of the Pu-F-Pu bridges.

Figure 2

Integrated ${}^{19}\mathrm{F}$ NMR signal intensities vs delay time for ${\mathrm{PuF}}_4$ measured at 2.35 T. Measurements at the two indicated frequencies in the inhomogeneously broadened line shape are shown, along with fits to a single-exponential recovery function.

Figure 3

Fluorine-19 spectra of ${\mathrm{PuF}}_4$(s). Experimental spectral intensities were measured as a function of the spectrometer carrier frequency at magnetic fields of 7.05 T (a) and 2.35 T (b), and are indicated by red crosses. Solid curves in (a) and (b) are fits of the function in Eq. (2) to the experimental data; nonfitted features centered at $\sim 0$ ppm are from residual ${}^{19}\mathrm{F}$ background signal. Simulated spectra were computed with models that assume ${5f}^4$ (c) and $5f^3 6d^1$ (d) Pu electronic configurations (vide infra); these spectra have been centered at the same isotropic shift frequency as (a) and (b) (${\delta }_0=-1620$ ppm) symbolized by the dashed line. Note that at fixed field, energy increases from right to left on the $\delta$ scale.