Valence-band structure of the skutterudite compounds ${\mathrm{CoAs}}_3,$ ${\mathrm{CoSb}}_3,$ and ${\mathrm{RhSb}}_3$ studied by x-ray photoelectron spectroscopy

The valence-band structure of ${\mathrm{CoAs}}_3,$ ${\mathrm{CoSb}}_3,$ and ${\mathrm{RhSb}}_3$ with a skutterudite-type crystal structure has been investigated by x-ray photoelectron spectroscopy. The photoemission spectra are compared with recent density-of-states calculations. Our photoemission spectra results and theoretical results are in good agreement for the energy positions in the metal d states and the pnicogen p states, but relatively large differences are found for the positions in the pnicogen s states. Based on our photoemission spectra, the electronic bonding states and the chemical trends are explained qualitatively in terms of a simple tight-binding model. The double localized and itinerant nature of the metal d states is also discussed in relation to the properties of the skutterudites. The metal d-derived density of states feature is clearly observed at about 1.2-, 1.4-, and 2.4-eV binding energies for ${\mathrm{CoSb}}_3,$ ${\mathrm{CoAs}}_3,$ and ${\mathrm{RhSb}}_3,$ respectively. From the point of view of the crystal-field effects, it can be considered that this d-character band originates predominantly from the d orbitals with $T_{2g}$ symmetry, while d orbitals with $E_g$ symmetry hybridize strongly with the p orbitals forming the conduction band. Since the $t_{2g}$ states are considered to be almost completely filled, corresponding to the zero spin $S=0\mathrm{}$ state ${(}^1{A}_1,t_{2g}^6),$ most of skutterudites exhibit diamagnetic properties. On the other hand, the slight chemical shifts of the core levels as compared with the pure elements indicate a small charge transfer from metal to pnicogen atoms in the skutterudites, leading to hybridization between metal d states and pnicogen p states. p-d hybridization causes not only a substantial screening of atomic Coulomb interactions at metal sites, but also a strong covalent bonding in these materials. Concerning a particular point of the band structure in skutterudites, our photoemission spectra near the valence-band edge show clear experimental evidence of a small density of states around the Fermi level due to a single band crossing the pseudogap.