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We have synthesized two new Eu-based compounds, EuSr2Bi2S4F4 and Eu2SrBi2S4F4which are derivatives of Eu3Bi2S4F4, an intrinsic superconductor with Tc = 1.5K. They belong to a tetragonal structure (SG: I4/mmm, Z = 2), similar to theparent compound Eu3Bi2S4F4. Our structural and 151Eu M\"ossbauer spectroscopystudies show that in EuSr2Bi2S4F4, Eu-atoms exclusively occupy thecrystallographic 2a-sites. In Eu2SrBi2S4F4, 2a-sites are fully occupied byEu-atoms and the other half of Eu-atoms and Sr-atoms together fully occupy4e-sites in a statistical distribution. In both compounds Eu atoms occupyingthe crystallographic 2a-sites are in a homogeneous mixed valent state ~ 2.6 -2.7. From our magnetization studies in an applied H = 9 Tesla, we infer thatthe valence of Eu-atoms in Eu2SrBi2S4F4 at the 2a-sites exhibits a shifttowards 2+. Our XPS studies corroborate the occurrence of valence fluctuationsof Eu and after Ar-ion sputtering show evidence of enhanced population ofEu2+-states. Resistivity measurements, down to 2 K suggest a semi-metallicnature for both compounds.

Unusual mixed valence of Eu in two new materials EuSr2Bi2S4F4 and Eu2SrBi2S4F4: M\"ossbauer and X-ray photoemission Spectroscopy investigations