Valence State of Eu and Superconductivity in Se-Substituted EuSrBiSF and EuSrBiSF.

Recently, we reported the synthesis and investigations of EuSrBiSF and EuSrBiSF. We have now been able to induce superconductivity in EuSrBiSF by Se substitution at the S site (isovalent substitution) with T = 2.9 K in EuSrBiSSeF. The other compound, EuSrBiSF, shows a significant enhancement of T. In Se-substituted EuSrBiSSeF, we find T = 2.6 K for x = 1.5 and T = 2.8 K for x = 2, whereas T = 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 Eu Mössbauer and X-ray photoemission spectroscopic measurements show that Se substitution in both of the compounds EuSrBiSF and EuSrBiSF gives rise to an increase in the Eu component in the mixed-valence state of Eu.