Critical evaluation of Eu valences from ${\mathit{L}}_{\mathrm{III}}$-edge x-ray-absorption and Mössbauer spectroscopy of ${\mathrm{EuNi}}_2$${\mathrm{Si}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$

In the quasiternary ${\mathrm{EuNi}}_2$ ${\mathrm{Si}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Ge}}_{\mathit{x}}$ 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 ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}$ Mössbauer 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 Mössbauer 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 Mössbauer 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.