High multipole transitions in NIXS: Valence and hybridization in 4f systems

Momentum-transfer (q) dependent non-resonant inelastic X-ray scattering measurements were made at the N4,5 edges for several rare-earth compounds. With increasing q, giant dipole resonances diminish, to be replaced by strong multiplet lines at lower energy transfer. These multiplets result from two different orders of multipole scattering and are distinct for systems with simple 4f and 4f initial states. A many-body theoretical treatment of the multiplets agrees well with the experimental data on ionic La and Ce phosphate reference compounds. Comparing measurements on CeO2 and CeRh3 to the theory and the phosphates indicates sensitivity to hybridization as observed by a broadening of 4f-related multiplet features. We expect such strong, non-dipole features to be generic for NIXS from f -electron systems. Copyright c © EPLA, 2008 The electronic, magnetic, and structural properties of rare-earth materials are strongly influenced by intraand inter-atomic interactions involving the 4f electrons. The problem of the local electronic structure of Ce provides a valuable case in point. Both Ce(4f1) and Ce(4f0) formally exist in ionic materials, but interaction between the 4f states and ligand or conduction states plays an important role in many-valence fluctuation or heavy-fermion cerium materials [1–11] such as Ce metal, CeRh3, CePd3, and CeO2, with cerium oxides in particular continuing to attract considerable attention for their catalytic properties [12–16]. The importance of hybridization in spite of the small radial extent of the 4f wave functions is due to the large 4f degeneracy. Further understanding of the 4f localization behavior in this class of materials is hampered not only by the theoretical difficulties presented by, e.g., competing interactions and strong hybridization, but also by the difficulty in obtaining and interpreting relevant spectroscopic information about the Ce local electronic structure. (a)E-mail: [email protected] Several core-shell spectroscopies have been applied to the 4d initial state in order to probe f -type final states in cerium compounds. At the Ce N4,5 edge, spin-orbit splitting is small, 4f -4d interaction is large [7], and the low binding energy of the 4d states compared to 3d or 2p orbitals results in a core-hole whose effect on 4f energy levels from reduced screening of the nuclear charge is lessened. X-ray absorption spectroscopy (XAS) measurements at the N4,5 edge however, contain both surface and bulk effects in their results and the need for a bulk-sensitive probe is understood [1]. Weak pre-edge multiplet features observed by XAS [4,6,8–10,17] at the N4,5 edge are due to the slight mixing of localized dipole-forbidden and delocalized dipole-allowed states [17]. Reflection-mode electron energy loss spectroscopy (EELS) at low incident electron energy has also been used to probe dipole and non-dipole transitions to 4f -type final states [18,19], but such measurements are also quite surface sensitive and may have additional complications due to multiple scattering. We revisit the phenomenon of excitations at the N4,5 edge in cerium compounds from a new perspective: nonresonant inelastic X-ray scattering (NIXS) [20,21]. Unlike