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Zusammenfassung:
We have synthesized two new Eu-based compounds, EuSr2Bi2S4F4 and Eu2SrBi2S4F4, which are derivatives of Eu3Bi2S4F4, an intrinsic superconductor with I-c = 1.5 K. They belong to a tetragonal structure (SG: I4/mmm, Z = 2), similar to the parent compound Eu3Bi2S4F4. Our structural and Eu-151 Mossbauer spectroscopy studies show that, in EuSr2Bi2S4F4, Eu-atoms exclusively occupy the crystallographic 2a -sites. In Eu2SrBi2S4F4, 2a -sites are fully occupied by Eu-atoms and the other half of Eu-atoms and Sr -atoms together fully occupy 4e -sites in a statistical distribution. In both compounds Eu atoms occupying the crystallographic 2a -sites are in a homogeneous mixed valent state similar to 2.6-2.7. From our magnetization studies in an applied H <= 9 T, we infer that the valence of Eu-atoms in Eu2SrBi2S4F4 at the 2a -sites exhibits a shift toward 2+. Our XPS studies corroborate the occurrence of valence fluctuations of Eu and after Ar-ion sputtering show evidence of enhanced population of Eu2+-states. Resistivity measurements, down to 2 K, suggest a semimetallic nature for both compounds.
