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Zusammenfassung:
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
Mossbauer spectroscopy suggests static mixed valency in EuPdP
corresponding to partially localized 4f 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 4f 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 4f 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.
