Synthesis, structural and physical properties of δ’-FeSe1-x

We report on synthesis, structural characterization, resistivity, magnetic and thermal expansion measurements on the as yet unexplored δ’-phase of FeSe1-x, here synthesized under ambient- (AP) and high-pressure (HP) conditions. We show that in contrast to β-FeSe1-x, monophasic superconducting δ’-FeSe1-x can be obtained in off-stoichiometric samples with excess Fe atoms preferentially residing in the van der Waals gap between the FeSe layers. The AP δ’-FeSe1-x sample studied here (Tc\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\simeq$\end{document} 8.5 K) possesses an unprecedented residual resistivity ratio RRR\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\simeq$\end{document} 16. Thermal expansion data reveal a small feature around ~90 K, which resembles the anomaly observed at the structural and magnetic transitions for other Fe-based superconductors, suggesting that some kind of “magnetic state” is formed also in FeSe. For HP samples (RRR\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\simeq$\end{document} 3), the disorder within the FeSe layers is enhanced through the introduction of vacancies, the saturated magnetic moment of Fe is reduced and only spurious superconductivity is observed.