Direct observation of valence transition in $\mathrm{Ce}{\mathrm{Ni}}_{1-x}{\mathrm{Co}}_x\mathrm{Sn}$ alloys by x-ray and photoelectron spectroscopies

We investigated the electronic structure of $\mathrm{Ce}{\mathrm{Ni}}_{1-x}{\mathrm{Co}}_x\mathrm{Sn}$ ($x=0.33$ and 0.38) as a function of temperature, combining x-ray absorption spectroscopy in the partial fluorescence yield, resonant x-ray emission spectroscopy, and photoelectron spectroscopy. The three spectroscopic techniques provided complementary and consistent information. For $x=0.33$, the valence transition into a mixed-valence regime is deduced to take place around $60\mathrm{K}$, as seen from the enhancement of the $f^0$ and $f^2$ components and the relative suppression of the $f^1$ component. The valence was stable for $x=0.38$ for temperatures down to $17\mathrm{K}$. The resonant inelastic x-ray scattering was also performed for $x=0.33$ at $17\mathrm{K}$, suggesting localized or partially delocalized $f$ states. Kondo peak was observed in the resonant valence spectra, showing the existence of the $c\text{−}f$ hybridization. The mechanism of the valence transition is discussed.

Figure 1

(Color online) Temperature dependence of the magnetic susceptibility of $\mathrm{Ce}{\mathrm{Ni}}_{1-x}{\mathrm{Co}}_x\mathrm{Sn}$.

Figure 2

(Color online) Temperature dependence of the PFY of $\mathrm{Ce}{\mathrm{Ni}}_{1-x}{\mathrm{Co}}_x\mathrm{Sn}$: (a) $x=0.33$ as-cast, (b) $x=0.38$ samples with the difference spectra between 17 and $300\mathrm{K}$, and (c) the comparison between $x=0.33$ and 0.38 at $17\mathrm{K}$.

Figure 3

Resonant inelastic scattering spectra for $x=0.33$ at $17\mathrm{K}$ with the PFY spectrum as a function of the incident photon energies.

Figure 4

(Color online) $f^1$, $f^2$, and fluorescence components as a function of the incident photon energies with the PFY spectrum. The intensity of $f^2$ is small and the vertical scale for $f^2$ is ten times as large as the scale of the other components.

Figure 5

(Color online) Temperature dependence of $\mathrm{Ce}L{\alpha }_1$ emission at the incident photon energy of $5717\mathrm{eV}$ for $\mathrm{Ce}{\mathrm{Ni}}_{1-x}{\mathrm{Co}}_x\mathrm{Sn}$ ($x=0.33$ and 0.38). Insets show enlarged parts of the $4f^2$ shoulder. The intensity is normalized to the maximum of the $f^1$ component.

Figure 6

(Color online) Relative intensity change in $f^2$ as a function of the temperature compared to the intensity at $300\mathrm{K}$.

Figure 7

(Color online) Absorption spectra (total electron yields) for $x=0.33$ at 27, 67, and $150\mathrm{K}$. The RPES is performed at the energies indicated by the arrow as will be shown later in Fig. 10.

Figure 8

(Color online) $\mathrm{Ce}3d$ photoelectron spectra for $x=0.33$ and 0.38 at 27 and $155\mathrm{K}$ at the incident photon energy of $1500\mathrm{eV}$. The energy positions of $f^n$ are conventionally assigned (not the calculated ones).

Figure 9

(Color online) Valence spectra for $x=0.33$ and 0.38 at 27, 67, and $155\mathrm{K}$ at the incident photon energy of $800\mathrm{eV}$.

Figure 10

(Color online) $\mathrm{Ce}3d\text{−}4f$ resonant photoelectron spectra as a function of the temperature for $x=0.33$. The A–E notations correspond to the energies indicated by the arrows in the absorption spectra of Fig. 7. The enlarged spectrum at the left-upper side of the figure is the high-resolution case at the Fermi edge for B.