Max-Planck Institut für Festkörperforschung - D-70506 Stuttgart, Germany;
Institut für Elektronische Eigenschaften Forschungszentrum D-52425 Jülich, Germany;
Institut für Anorganische Chemie und Strukturchemie II Heinrich-Heine-Universität - D-40225 Düsseldorf, Germany;
Institut für Physikalische Chemie II Universität zu Köln D-50939 Köln, Germany

Photoemission spectra and the TB-LMTO-ASA band structure of the mixed valent europium compound EuPdP (ZrBeSi type structure) were studied and compared with the results obtained on isostructural SrPdP and EuAuAs and the ThCr2Si2-type compound EuNi2P2. EuNi2P2 is a homogeneous mixed valent compound, whereas the situation in EuPdP is not clear. The Moessbauer spectroscopy suggests static mixed valency in EuPdP corresponding to partially localized 4 f electrons, but from the photoemission measurement we know that the situation in EuPdP is different from the static mixed valent Eu3O4. Band structure calculations as well as photoemission spectra indicate a 4 f density of states in the vicinity of the Fermi level in EuPdP, which belongs to a van Hove singularity (vHS). In EuPdP we therefore expect homogeneous mixed valence, similar to EuNi2P2 with itinerant 4 f electrons. The absence of homogeneous mixed valence is possibly associated with the layered structure of EuPdP, enabling charge ordering through a martensitic phase transition. A high density of states associated with a singularity at the Fermi energy seems to be responsible for the structural instability in inhomogeneous mixed valent europium compounds.

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Europhysics Letters, 40 85-91, 1997.