Structural and superconductivity study on α-FeSex

We have successfully synthesized the alpha-FeSex binary tetragonal superconductors with nominal composition of FeSex (x=0.6-1.0) via conventional solid state reactions between Fe and Se sealed in quartz tubes. Fe and beta-FeSe are the most commonly seen impurities in this binary system. A low-temperature annealing at 400 degrees C is found to be crucial to remove beta-FeSe, which is the thermodynamic stable phase with hexagonal symmetry. For all the samples of FeSex, superconductivity is confirmed by magnetic measurements as well as resistivity measurements with their T-c at around 8 K. We noticed that their T-c does not vary with the different nominal Se amount. High-resolution synchrotron X-ray diffraction analysis revealed that the unit cell parameters of all these samples do not change within the error range, and their structure only tolerate the same very small amount of Se deficiency. Based on this study, we concluded that the alpha-FeSex superconductor only exist in a very narrow deficiency range. (C) 2009 Elsevier Ltd. All rights reserved.