Valence State of Eu and Superconductivity in Se-Substituted EuSr₂Bi₂S₄F₄ and Eu₂SrBi₂S₄F₄

Recently, we reported the synthesis and investigations of EuSr₂Bi₂S₄F₄ and Eu₂SrBi₂S₄F₄. We have now been able to induce superconductivity in EuSr₂Bi₂S₄F₄ by Se substitution at the S site (isovalent substitution) with T_c = 2.9 K in EuSr₂Bi₂S₂Se₂F₄. The other compound, Eu₂SrBi₂S₄F₄, shows a significant enhancement of T_c. In Se-substituted Eu₂SrBi₂S_4-xSe_xF₄, we find T_c = 2.6 K for x = 1.5 and T_c = 2.8 K for x = 2, whereas T_c = 0.4 K in the Se-free sample. In addition to superconductivity, an important effect associated with Se substitution is that it gives rise to remarkable changes in the Eu valence. Our ¹⁵¹Eu Mössbauer and X-ray photoemission spectroscopic measurements show that Se substitution in both of the compounds Eu₂SrBi₂S₄F₄ and EuSr₂Bi₂S₄F₄ gives rise to an increase in the Eu²⁺ component in the mixed-valence state of Eu.