Unusual Mixed Valence of Eu in Two Materials-EuSr 2 Bi 2 S 4 F 4 and Eu 2 SrBi 2 S 4 F 4 : Mössbauer and X-ray Photoemission Spectroscopy Investigations.

We have synthesized two new Eu-based compounds, EuSr2 Bi2 S4 F4 and Eu2 SrBi2 S4 F4 , which are derivatives of Eu3 Bi2 S4 F4 , an intrinsic superconductor with Tc = 1.5 K. They belong to a tetragonal structure (SG: I4/mmm, Z = 2), similar to the parent compound Eu3 Bi2 S4 F4 . Our structural and151 Eu Mössbauer spectroscopy studies show that, in EuSr2 Bi2 S4 F4 , Eu-atoms exclusively occupy the crystallographic 2a-sites. In Eu2 SrBi2 S4 F4 , 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 ∼2.6-2.7. From our magnetization studies in an applied H ≤ 9 T, we infer that the valence of Eu-atoms in Eu2 SrBi2 S4 F4 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.