Europium silicides and germanides of the EuM₂X₂ type. Crystal structure and the valence states of europium

X-ray diffraction studies of EuM₂X₂ compounds (M = Fe, Co, Ni, Cu; X = Si, Ge) revealed that these compounds crystallize in the ThCr₂Si₂ type body-centered tetragonal structure, with the space group I4 mmm. Distribution of the atoms among the lattice sites, the free parameter of the Si and Ge atoms, and the interatomic distances of the compounds were determined by crystal structure calculations. Europium, as determined by Mössbauer spectroscopy, is present in the di- and tri-valent states in the EuM₂Si₂ compounds, the relative amounts of the two states being different in each of the compounds. In EuNiSi₃, EuNi₂Ge₂ and EuCu₂Ge₂ all the Eu are divalent. The relationship between the structure properties and the valence states of Eu in the compounds is discussed.