CRITICAL-EVALUATION OF EU VALENCES FROM LIII-EDGE X-RAY-ABSORPTION AND MOSSBAUER-SPECTROSCOPY OF EUNI2SI2-XGEX

In the quasiternary EuNi2Si2-xGe(x) system most of the Eu ions exhibit an intermediate-valence character, depending on the number of Si neighbors and the temperature. A detailed study of EuNi2Si0.5Ge1.5 by Eu L(III) x-ray-absorption and Eu-151 Mossbauer spectroscopy reveals a sharp temperature-induced valence change, which is more complete than in any other Eu system. A linear correlation between the mean Eu valence, upsilon-(L(III), derived from L(III)-edge measurements, and the mean Mossbauer isomer shift is only observed if a variation of the recoil-free fraction at the different Eu sites is taken into account. The Eu L(III) spectra of EuNi2Ge2, in which the Eu is divalent according to Mossbauer and magnetization measurements, result in upsilon-(L(III)) = 2.15, which is similar to other divalent intermetallic Eu systems. This observation is attributed to final-state effects due to covalency. The magnetic susceptibility of EuNi2Si0.5Ge1.5 is in accord with the intermediate-valence character of this compound, which orders ferromagnetically at 25 K.