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Zusammenfassung:
Photoemission spectra and TB-LMTO-ASA band structure calculations of
some mixed valency europium compounds have been studied. The band
structures are compared with the band structures of the isostructural
lanthanum and strontium compounds. Surprisingly a 4f density of states
in the vicinity of the Fermi level is observed in inhomogeneous mixed
valency EuPd3B, Eu3S4, and EuPdP. Indeed a van Hove Singularity (vHS)
derived from the d states of La and Pd or p states of boron or
phosphorous are found in La3S4, LaPd3B and SrPdP. The valence
instability in the Eu compounds is thus not necessarily due to Eu 4f
states. The results also provide some ground for the assumption that
inhomogeneous mixed valency is an indicator for a vHS in the conduction
band. The europium compounds order anti-ferromagnetically at low
temperatures, whereas La3S4 becomes superconducting at 8 K. In La3S4,
the vHS coincides with the Fermi level, whereas the singularity lies
below the Fermi level in SrPdP and LaPd3B. According to the van Hove
scenario it must be possible to induce superconductivity via hole doping
in SrPdP and LaPd3B. (C) 1997 Elsevier Science S.A.
