Unusual mixed valence of Eu in two materials —EuSr₂Bi₂S₄F₄and Eu₂SrBi₂S₄F₄: Mössbauer and x-ray photoemission spectroscopy investigations

Abstract

We have synthesized two new Eu-based compounds, EuSr₂Bi₂S₄F₄ and Eu₂SrBi₂S₄F₄, which are derivatives of Eu₃Bi₂S₄F₄, an intrinsic superconductor with T_c = 1.5 K. They belong to a tetragonal structure (SG: I4/mmm, Z = 2), similar to the parent compound Eu₃Bi₂S₄F₄. Our structural and ¹⁵¹Eu Mössbauer 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 ∼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.