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 $\rm ThCr_2Si_2$-type compound $\rm EuNi_2P_2$. $\rm EuNi_2P_2$ is a homogeneous mixed valent compound, whereas the situation in EuPdP is not clear. The Mössbauer 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 $\rm Eu_3O_4$. 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 $\rm EuNi_2P_2$ 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.