Tuning the valence in ternary Eu-pnictides: the series EuPd_1-xAg_xP and EuPd_1-xAu_xAs

The series of ternary Eu-pnictides EuPd1-xAgxP and EuPd1-xAuxAs were synthesized by heating the elements and characterized by means of X-ray powder diffraction. The unusual behaviour of the lattice constants are explained by geometric and electronic factors, in particular the change of the Eu valence. In EuPdP and EuPdAs, the Eu atoms adopt a temperature-dependent mixed valent state which tends towards divalency while substituting Pd by Ag or Au. This is investigated by means of temperature-dependent X-ray diffraction, TB-LMTO-ASA band structure calculations and resonant photoemission (PE) experiments using synchrotron radiation. The calculated density of states (DOS) was compared with the results 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 are expected. The results of the LMTO band calculations are in good agreement with the experimental facts, leading to the conclusion that the range of valence instabilities of europium is limited to x<0.4 in EuPd1-xAgxP and x