Localized 5f² states in UPd₅Al₂ and valence crossover in the vicinity of heavy-Fermion superconductivity

Inelastic neutron scattering experiments have been carried out to reveal the 5 f electron states of UPd₅Al₂, an isostructural compound of the heavy-fermion superconductor NpPd₅Al₂. The tetravalent U⁴⁺ with localized 5 f ² configuration has been revealed from the sharp crystalline electric field (CEF) excitations at 8.5 and 15.5 meV with a similar level scheme to PrPd₅Al₂. The absence of a quasi-elastic response is evidence for the singlet states only realized in the 5 f ² non-Kramers ion. The large splitting and significant broadening at high temperatures are the characteristic of 5 f electrons with large orbitals. The magnetic susceptibility is reasonably explained with the mean-field model based on the low-lying levels, where the wave functions have been determined from the neutron spectra. The present study involves the valence crossover in AnPd₅Al₂ from tetravalent U⁴⁺ to trivalent Np³⁺. This valence crossover can be recognized from an abnormal lattice expansion and large electrical resistivity.