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 UPd3 and heavy fermion UPd2Al3 as a function of pressure. The spectral line shape of the absorption edge is found to vary rapidly in UPd2Al3 with a notable broadening of the white line above the structural transition around 25 GPa while it shows a more gradual variation in UPd3 over the considered pressure range (0-40 GPa), indicating different responses of the f-d electrons to the compressed lattice in the two compounds. The U L-3 absorption spectra in both UPd3 and UPd2Al3 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 UPd3 which shows a remarkably stable valency under pressure, close the nominal value of 4. On the contrary, our results point to a mixed valent U4-delta ground state in UPd2Al3 at ambient conditions, evolving into a U4+ (or possibly U4+delta) configuration at high pressure. The f-electron delocalization could be responsible for the known structural transition in UPd2Al3.