Probing the nature of the $\mathrm{Ce}4f$ states in $\mathrm{Ce}{X}_9{\mathrm{Si}}_4(X=\mathrm{Ni},\mathrm{Co})$ by high-energy electron spectroscopies

We have used x-ray photoelectron spectroscopy, and resonant inverse photoemission at the $\mathrm{Ce}{M}_5(3d_{5∕2}\to 4f)$ edge, to study the $\mathrm{Ce}4f$ states in the isostructural metallic compounds ${\mathrm{CeCo}}_9{\mathrm{Si}}_4$ and ${\mathrm{CeNi}}_9{\mathrm{Si}}_4$. The spectroscopic data reveal the intermediate-valence nature of these materials. They also suggest that a larger density of states at the Fermi level, and a reduced $4f$ energy, are the origin of an enhanced $4f$–conduction band hybridization in the Co compound. This provides an explanation for the remarkable differences in the magnetic, transport, and thermodynamic properties of the two materials.

Figure 1

PES spectra of the $\mathrm{Ce}3d$ core levels of ${\mathrm{CeNi}}_9{\mathrm{Si}}_4$, ${\mathrm{CeCo}}_9{\mathrm{Si}}_4$, and of selected IV Ce compounds. (a) ${\mathrm{Ce}}_7{\mathrm{Ni}}_3$, (Ni), ${\mathrm{CeNi}}_9{\mathrm{Si}}_4$; (b) $\mathrm{CeNi}$; (Co), ${\mathrm{CeCo}}_9{\mathrm{Si}}_4$; (c) ${\mathrm{CeNi}}_2$; (d) ${\mathrm{CeRh}}_3$. Vertical ticks mark the various final states (thick $3d_{3∕2}$; thin $3d_{5∕2}$).

Figure 2

Valence band PES spectra of ${\mathrm{CeNi}}_9{\mathrm{Si}}_4$ and ${\mathrm{CeCo}}_9{\mathrm{Si}}_4$ measured at $300\mathrm{K}$ $(h\nu =1486\mathrm{eV})$. The arrow marks the “ionization” $\left(f^0\right)$ final state in ${\mathrm{CeNi}}_9{\mathrm{Si}}_4$. The inset shows a closeup of the spectra near the Fermi level.

Figure 3

RIPES of ${\mathrm{CeCo}}_9{\mathrm{Si}}_4$ near the $\mathrm{Ce}3d_{5∕2}\left(M_5\right)$ threshold. Spectra measured at increasing incident energy $E_{in}=E^{*}+881\mathrm{eV}$ have been vertically offset for clarity.

Figure 4

(a): RIPES spectra of ${\mathrm{CeNi}}_9{\mathrm{Si}}_4$ measured at $80\mathrm{K}$ at the maxima of the $f^1$ $(E^{*}=0)$ and $f^2$ $(E^{*}=3\mathrm{eV})$ resonances. (b) Comparison of the $E^{*}=0$ spectra at 80 and $300\mathrm{K}$.

Figure 5

Constant final state (CFS) curves of (a) ${\mathrm{CeCo}}_9{\mathrm{Si}}_4$ and (b) ${\mathrm{CeNi}}_9{\mathrm{Si}}_4$ showing the separate $f^1$ and $f^2$ resonance profiles. CFS curves measured at 80 and $300\mathrm{K}$ have been arbitrarily normalized to the maximum of the $f^2$ resonance.