TUNING THE VALENCE IN TERNARY EU-PNICTIDES - THE SERIES EUPD1-XAGXP AND EUPD1-XAUXAS

The series of ternary Eu-pnictides EuPd1-xAgxP and EuPd1-xAuxAs were s ynthesized by heating the elements and characterized by means of X-ray powder diffraction. The unusual behaviour of the lattice constants ar e explained by geometric and electronic factors, in particular the cha nge of the Eu valence. In EuPdP and EuPdAs, the Eu atoms adopt a tempe rature-dependent mixed valent state which tends towards divalency whil e substituting Pd by Ag or Au. This is investigated by means of temper ature-dependent X-ray diffraction, TB-LMTO-ASA band structure calculat ions and resonant photoemission (PE) experiments using synchrotron rad iation. The calculated density of states (DOS) was compared with the r esults of the PE experiments. The measured partial DOS, as well as the LMTO-DOS, of EuPdP is marked by a high DOS at the Fermi level. A van Hove singularity in the band structure leads to a logarithmic DOS-peak . If the Fermi level coincides with this peak, valence instabilities a re expected. The results of the LMTO band calculations are in good agr eement with the experimental facts, leading to the conclusion that the range of valence instabilities of europium is limited to x<0.4 in EuP d1-xAgxP and x<0.15 in EuPd1-xAuxAs.