Pressure and temperature dependence of the Eu valence in $EuNi_2Ge_2$ and related systems studied by Mössbauer effect, X-ray absorption and X-ray diffraction

The effect of pressure and temperature on the valence state of Eu in EuNi2Ge2 systems and the related systems EuNi2Si2, EuNi2Si0.5Ge1.5 and EuPl2Ge2 was studied by 151Eu-Mössbauer spectroscopy, EuLIII X-ray absorption and X-ray diffraction. Pressure-induced valence transitions from a divalent towards a trivalent state were observed in EuNi2Ge2 around 5 GPa and in the quasi-ternary EuNi2Si0.5Ge1.5 system around 0.5 GPa. In EuNi2Ge2 we found from Mössbauer spectra measured at various pressures and temperatures a new type of valence transition, where an initially homogeneous valence state near to divalency segregates into two different valent states. From these data we derive for EuNi2Ge2 the pressure and temperature dependence of the Eu valence and magnetism