Europium valency in EuCu_2−xFe_xSi₂, EuCu₂Si_2−xGe_x, EuCu_2−xSi_{2+ x} and EuNi_2−xSi_{2+ x}

Mössbauer spectroscopy, X-ray photoemission spectroscopy and magnetization studies of EuCu_2−xFe_xSi₂, EuCu₂Si_2−xGe_x, EuCu_2−xSi_{2+ x}, and EuNi_2−xSi_{2+ x} have been performed. In EuFe₂Si₂ and EuNi₂Si₂ the Eu ion is trivalent, in EuCu₂Si₂ it is of intermediate valency. In all systems with x=0, many inequivalent Eu sites of intermediate valency are formed. In EuCu_2−xFe_xSi₂, the average valency of Eu moves nonmonotonically toward Eu³⁺. In EuCu₂Si_2−xGe_x, EuCu_2−xSi_{2+ x} and EuNi_2−xSi_{2+ x}, the Eu average valency moves quickly towards Eu²⁺. In all three systems, when x=1, all Eu ions are already stable divalent. The systems EuCuSi₃ and EuNiSi₃ order magnetically at 40 K and 34 K, and exhibit hyperfine fields of 323 kOe and 458 kOe, respectively. XPS and Mössbauer isomer shift determinations of the average Eu valence as a function of temperature in EuCu_1.5Fe_0.5Si₂ are in good agreement. The experimental observations concerning the Eu valencies can not be explained solely in terms of the local volume available to the Eu ion; the nature of chemical environment plays a dominant role.