f-Electron states in PrPd₅Al₂

The crystalline electric field (CEF) excitation spectra of PrPd₅Al₂ have been studied by neutron inelastic scattering in order to reveal the f-electron states, where PrPd₅Al₂ is a Pr-based isostructural compound of the heavy-fermion superconductor NpPd₅Al₂. We observed clear CEF excitations up to ∼25 meV. The CEF Hamiltonian H_CEF of the Pr³⁺ ion (³H₄) under tetragonal point symmetry was orthogonalized analytically and the CEF parameters were determined from the excitation energies. The f-electron states reproduce the distinctive temperature dependence of the magnetic excitation spectra as well as the macroscopic properties. A point charge model is effective for understanding the systematic change in the f-electron states in RPd₅Al₂. The Ising anisotropy in CePd₅Al₂, PrPd₅Al₂, and NdPd₅Al₂ originates from the flat orbitals with large J_z due to the CEF potential developed in the crystal structure. We found that the temperature dependence of the magnetic susceptibility in NpPd₅Al₂ can be explained qualitatively on the basis of the identical charge distribution in RPd₅Al₂. XY-type anisotropy is expected from the positive Stevens factors α_l for the Np³⁺ ion. Thus, we conclude that the local property is important in NpPd₅Al₂, whose origin is common in the RPd₅Al₂ isostructural family.