A new view of valence instabilities in europium compounds: van Hove singularity in EuPdP

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.