Europium valencies in substituted EuCu2Si2 systems

Mössbauer studies of¹⁵¹Eu in EuCu_2?x Pt _x Si₂, EuCu₂Si_2?x Ge _x and Eu _x Gd_1?x Mn₂Si₂ (x≤0.25) at temperatures between 90 K and 425 K have been performed. All spectra are composed of at least two subspectra, corresponding to stable Eu²⁺ and intermediate valent Eu. The isomer shift of the stable divalent Eu (ordered magnetically up to 400 K in Eu_0.25Gd_0.75Mn₂Si₂) isS ₂=?7.5 mm/sec in comparison toS ₂=?10.5 mm/s for stable Eu²⁺ in EuMn₂Ge₂. This shows that the Eu²⁺ is under heavy internal pressure in these systems. From the temperature dependence of the hyperfine interaction parameters and line intensities conclusions are derived concerning interconfigurational excitation energies, their inhomogeneous width and recoil free fraction behaviour.