Influence Of Optimized Synthesis And Sn Addition In “11” Type Iron-based Superconductors

The new iron-based superconductor discovered in 2008 has induced an explosive research boom in the field of condensed-matter after the famous copper oxides superconductors.As the promising one in the novel iron-based superconductors,“11” type iron-based superconductors are choosed as the focus of this area because of their simple structure,low toxicity and doping sensitivity.Since the sintering temperature at second step of solid-state reaction plays a vital role in the evolution of crystal structure and microstructure,the correlation between crystal structure and superconductivity motivated by various sintering temperature of FeSe0.5Te0.5 polycrystalline samples has also been investigated.The crystal structure analysis indicates elevated sintering temperature at second step can increase chemical pressure by facilitating the substitution of Te into the superconducting phase,which contributes to the increase of the onset temperature of superconducting transition(Tc onset).Meanwhile,the accelerated growth of the superconducting grains and the promoted homogeneity motivated by elevated sintering temperature serve as the main reason for the sharp superconducting transition.In present work,we investigated the effect of Sn addition on FeSe0.93 and FeSe0.5Te0.5 bulks,which were prepared by solid-state reaction in Ar atmosphere.The result suggests that Sn doesn't actually enter the crystal lattice of the superconducting phase,existing as an inclusion instead.According to the thermal analysis,Sn was firstly melt and then reacted with Se/Te forming SnSexTey during the sintering process.The presence of liquid SnSexTey can obviously accelerate the subsequent formation of iron selenide.X-ray diffraction(XRD)spectrum indicates that Sn addition can also expedite the transformation from Fe7Se8 to the superconducting phase and promote the growth of the superconduting grains.Morphology observation also displays enlarged superconducting grains and their better connection with each other in the Sn added samples.Although the existence of impurities such as Fe7Se8,Fe3O4 and SnSexTey is harmful to the superconductivity of “11” type iron-based superconductors,the superconducting transition temperature(Tc)of the Sn doped FeSe0.93 samples isn't suppressed compared to the undoped one Owing to the accelerated growth of the superconducting grains,decreased detrimental phase and the overall homogeneity,quite different from previous studies on the metal doped FeSe superconductors.Besides,minor Sn addition of the FeSe0.5Te0.5 bulks bulks facilitates the substitution of Te on Se-Site in the superconducting phase whose composition is closer to the ideal one(FeSe0.5Te0.5).5 wt% Sn addition dramatically enhances the zero resistivity temperature(Tc0)by 3 K accompanying with a bit improvement in Tc onset.The critical current density(Jc)of the 5 wt% Sn-added sample maintains a stable value of 3×102 A cm-2 even beyond 60000 Oe at 4.3 K,unlike the quick decay in the Sn-free sample.Proper Sn addition is a promising way to improve the superconducting performance of FeSe0.5Te0.5 superconductor.