High‐Purity FeSe1−x Superconductors Prepared by Solid‐State Synthesis and Liquid Phase Processing

Abstract

FeSe1−x samples have been prepared by a solid‐state reaction in the Ar+3% H2 gas flow. The samples sintered at 410°C were of the tetragonal PbO structure (β‐phase), showing little trace of δ‐phase (hexagonal). However, the samples sintered at higher temperatures contained a fair amount of δ‐phase, which was unable to be eliminated completely by subsequent annealing at ≤450°C for a prolonged period. Both M–T and R–T measurements showed that sintered samples of β‐phase were superconducting with an onset Tc above 8.1 K. Thick films of FeSe1−x were grown on LaAlO3 substrates by liquid phase processing with SeSn as the flux. In contrast to the sintering, the films grown around 910°C from the Sn‐containing liquid had a pure β‐phase. Although the phase diagram showed some solid solubility of Sn in FeSe, energy‐dispersive X‐ray spectroscopy failed to find Sn trace in the crystallized films, which showed a steeper superconducting transition at lower Tc of 6.0 K. To clarify whether the possible Sn contamination caused the drop of Tc, FeSe1−x:5% Sn samples were sintered, which showed little change of Tc from the pure β‐FeSe1−x. The cause for the Tc reduction of thick films remains unclear.