Pressure-driven valence change in ternary Eu pnictides

We observed a structural phase transition with extremely anisotropic changes of the lattice parameters as a function of pressure at 2.6 GPa in EuPdP, which crystallizes in the hexagonal layered structure type. On the basis of the results of pressure-dependent x-ray diffraction experiments on the isostructural series APdP and APdAs (A = Sr or a trivalent rare-earth element) we show that the phase transition in EuPdP is accompanied by a valence change of the Eu. Strong but continuous changes of the lattice parameters with increasing pressure, which are due to increase of the Eu valence, were observed in EuNiP, EuPtP and EuPdAs, too. An estimation of the average Eu valence in these compounds leads to preferred values of the order of 2 n/6

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<0.15 in EuPd1-xAuxAs

First- and second-order phase transitions in ternary europium phosphides with ThCr2Si2\mathrm{ThCr_2Si_2}-type structure

The ThCr2Si2- type compounds EuFe2P2 and EuRu2P2 exhibit as a function of pressure continuous phase transitions which are accompanied by extremely strong changes of the lattice parameters and particularly of the P–P distance dP−P along the tetragonal c-axis. In the isostructural integralvalent LaT2P2 compounds similar phase transitions of second (T=Fe) and first order (T=Co) occur as a function of pressure indicating that the T element essentially determines the nature of the phase transition. The different types of phase transitions are explained phenomenologically

First-order phase transitions in EuCo2P2\mathrm{EuCo_2P_2} and SrNi2P2\mathrm{SrNi_2P_2}

First-order phase transitions with strong and extremely anisotropic changes of the lattice parameters were observed in the ThCr2Si2 structure-type compounds EuCo2P2 and SrNi2P2. At room temperature, with increasing pressure the phase transition occurs in SrNi2P2 at 4 kbar and in EuCo2P2 at 30 kbar which is in the latter probably accompanied by a valence change of Eu. On the basis of single-crystal data of ACo2P2 (A = Ca, Sr, La, Ce, Pr, Nd, Eu) at ambient pressure and temperature we discuss the pressure dependence of the bond lengths in these compounds

First-order phase transitions in the ThCr2Si2\mathrm{ThCr_2Si_2}-type phosphides ARh2P2\mathrm{ARh_2P_2} (A = Sr, Eu)

We observed for the first time a first-order phase transition with strong and extremely anisotropic changes of the lattice parameters in compounds crystallizing in the ThCr2Si2 structure type. The phase transition occurs SrRh2P2 with increasing pressure at 6 GPa (300 K) and in EuRh2P2 with increasing temperature at 810 K (ambient pressure). On the basis of single-crystal data of ARh2P2 (A = Ca, Sr, Ba, Eu) at ambient pressure and temperature we discuss the PP distance in the framework of band-structure calculations at the first-order phase transition the PP state changes from a ‘no-bond’ to a ‘single-bond’ state