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 an d compared with the results obtained on isostructural SrPdP and EuAuAs and the ThCr2Si2-type compound EuNi2P2. EuNi2P2 is a homogeneous mixe d valent compound, whereas the situation in EuPdP is not clear. The Mo ssbauer spectroscopy suggests static mixed valency in EuPdP correspond ing to partially localized 4f electrons, but from the photoemission me asurement we know that the situation in EuPdP is different from the st atic mixed valent Eu3O4 Band structure calculations as well as photoem ission spectra indicate a 4f density of states in the vicinity of the Fermi level in EuPdP, which belongs to a van Hove Singularity (vHS). I n EuPdP we therefore expect homogeneous mixed valence, similar to EuNi 2P2 with itinerant 4f electrons. The absence of homogeneous mixed vale nce is possibly associated with the layered structure of EuPdP, enabli ng charge ordering through a Martensitic phase transition. A high dens ity of states associated with a singularity at the Fermi energy seems to be responsible for the structural instability in inhomogeneous mixe d valent europium compounds.