Pressure-induced f-electron delocalization in the U-based strongly correlated compounds UPd3 and UPd2Al3: Resonant inelastic x-ray scattering and first-principles calculations

We have measured the uranium ${L}_{3}$ absorption and resonant emission spectra in the localized magnetic compound $\mathrm{U}{\mathrm{Pd}}_{3}$ and heavy fermion $\mathrm{U}{\mathrm{Pd}}_{2}{\mathrm{Al}}_{3}$ as a function of pressure. The spectral line shape of the absorption edge is found to vary rapidly in $\mathrm{U}{\mathrm{Pd}}_{2}{\mathrm{Al}}_{3}$ with a notable broadening of the white line above the structural transition around $25\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ while it shows a more gradual variation in $\mathrm{U}{\mathrm{Pd}}_{3}$ over the considered pressure range $(0\char21{}40\phantom{\rule{0.3em}{0ex}}\mathrm{GPa})$, indicating different responses of the $f\text{\ensuremath{-}}d$ electrons to the compressed lattice in the two compounds. The U ${L}_{3}$ absorption spectra in both $\mathrm{U}{\mathrm{Pd}}_{3}$ and $\mathrm{U}{\mathrm{Pd}}_{2}{\mathrm{Al}}_{3}$ and their pressure dependence were further simulated via first-principles band calculations within the linear muffin-tin orbital approach. The calculations reproduce the main features of the experimental absorption edges. The calculated pressure dependence of the $f$ charge reveals a stronger localization of the $f$ electrons in $\mathrm{U}{\mathrm{Pd}}_{3}$ which shows a remarkably stable valency under pressure, close the nominal value of 4. On the contrary, our results point to a mixed valent ${\mathrm{U}}^{4\ensuremath{-}\ensuremath{\delta}}$ ground state in $\mathrm{U}{\mathrm{Pd}}_{2}{\mathrm{Al}}_{3}$ at ambient conditions, evolving into a ${\mathrm{U}}^{4+}$ (or possibly ${\mathrm{U}}^{4+\ensuremath{\delta}}$) configuration at high pressure. The $f$-electron delocalization could be responsible for the known structural transition in $\mathrm{U}{\mathrm{Pd}}_{2}{\mathrm{Al}}_{3}$.