Critical evaluation of Eu valences from LIII-edge x-ray-absorption and Mössbauer spectroscopy of EuNi2Si2-xGex.

Abstract

In the quasiternary ${\mathrm{EuNi}}_{2}$${\mathrm{Si}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ge}}_{\mathit{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 ${\mathrm{EuNi}}_{2}$${\mathrm{Si}}_{0.5}$${\mathrm{Ge}}_{1.5}$ by Eu ${\mathit{L}}_{\mathrm{III}}$ x-ray-absorption and $^{151}\mathrm{Eu}$ M\"ossbauer 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, v(${\mathit{L}}_{\mathrm{III}}$), derived from ${\mathit{L}}_{\mathrm{III}}$-edge measurements, and the mean M\"ossbauer isomer shift is only observed if a variation of the recoil-free fraction at the different Eu sites is taken into account. The Eu ${\mathit{L}}_{\mathrm{III}}$ spectra of ${\mathrm{EuNi}}_{2}$${\mathrm{Ge}}_{2}$, in which the Eu is divalent according to M\"ossbauer and magnetization measurements, result in v(${\mathit{L}}_{\mathrm{III}}$)=2.15, which is similar to other divalent intermetallic Eu systems. This observation is attributed to final-state effects due to covalency. The magnetic susceptibility of ${\mathrm{EuNi}}_{2}$${\mathrm{Si}}_{0.5}$${\mathrm{Ge}}_{1.5}$ is in accord with the intermediate-valence character of this compound, which orders ferromagnetically at 25 K.