Valence instability in YbPd₂Si₂: magnetic susceptibility, X-ray absorption, and photoemission studies

Abstract

The bulk and surface valence states of Yb were determined for the intermetallic compound YbPd₂Si₂ by magnetic susceptibility (χ), L_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 χ at about 30 K, and Curie-Weiss behavior at high temperatures. The observed temperature dependence of χ 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_III-edge x-ray absorption spectra, which reveal a weak temperature dependence of the Yb mean valence v̅ in agreement with the results from an analysis of the χ 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 YbPd₂Si₂ is close to the high-temperature limit given by the ionic ICF model.