Valence instability in YbPd2Si2: Magnetic susceptibility, x-ray absorption, and photoemission studies

The bulk and surface valence states of Yb were determined for the intermetallic compound Yb${\mathrm{Pd}}_2$ ${\mathrm{Si}}_2$ by magnetic susceptibility ($\chi$), $L_{\mathrm{III}}$-edge x-ray absorption, and valence-band photoemission measurements. In the susceptibility data this compound exhibits the characteristic features of homogeneously mixed-valent systems: enhanced Pauli paramagnetism at low temperatures, a maximum in $\chi$ at about 30 K, and Curie-Weiss behavior at high temperatures. The observed temperature dependence of $\chi$ is discussed in light of interconfigurational fluctuation (ICF) as well as Fermi-liquid models. The mixed-valent state of Yb is also borne out in the $L_{\mathrm{III}}$-edge x-ray absorption spectra, which reveal a weak temperature dependence of the Yb mean valence $\overline{v}$ in agreement with the results from an analysis of the $\chi$ data in light of the ICF model. Valence-band photoemission spectra support our view of a homogeneously mixed-valent state of Yb; they further identify a valence transition to the divalent state in the outermost surface layer. $4f$ binding energies are found to be 0.80±0.05 eV higher at the surface as compared to the bulk. All three methods suggest that—within the limits of accuracy—the room-temperature mean valence of Yb in Yb${\mathrm{Pd}}_2$ ${\mathrm{Si}}_2$ is close to the high-temperature limit given by the ionic ICF model.