PRESSURE AND TEMPERATURE-DEPENDENCE OF THE EU VALENCE IN EUNI2GE2 ANDRELATED SYSTEMS STUDIED BY MOSSBAUER-EFFECT, X-RAY-ABSORPTION AND X-RAY-DIFFRACTION

The effect of pressure and temperature on the valence state of Eu in E uNi2Ge2 systems and the related systems EuNi2Si2, EuNi2Si0.5Ge1.5 and EuPt2Ge2 was studied by Eu-151-Mossbauer spectroscopy, Eu-L(III) X-ray absorption and X-ray diffraction. Pressure-induced valence transition s 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 Mossbauer 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 t he pressure and temperature dependence of the Eu valence and magnetism .