Abstract
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 (T c \(\simeq\) 8.5 K) possesses an unprecedented residual resistivity ratio RRR \(\simeq\) 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 \(\simeq\) 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.
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