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≃ 8.5\,K) possesses an unprecedented residual resistivity ratio RRR ≃ 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. %indicative of a fluctuating magnetic ordering. For HP samples (RRR ≃ 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.
@article{arxiv.1006.0073,
title = {Synthesis, structural and physical properties of $\delta'$-FeSe$_{1-x}$},
author = {Mariano de Souza and Amir-Abbas Haghighirad and Ulrich Tutsch and Wolf Assmus and Michael Lang},
journal= {arXiv preprint arXiv:1006.0073},
year = {2010}
}