Characterization Of Iron Chalcogenide Superconductors Prepared By Mechanical Ball-milling And Low-temperature Synthesis

The discovery of new iron-based LaO1xFxFeAs in the early2008was found toexhibit superconductivity and this break-through discovery soon triggered atremendous research interest in condensed matter physics. Of all the superconductingmaterial types, iron-based superconductor is an untraditional one, which is unable tobe understood in the light of BCS (Bardeen Cooper Schieffer) theory. According tothe composition and crystal structure of the parent compound, iron-basedsuperconductor currently can be classified into two major families: iron arsenides andiron chalcogenides. Therefore, due to the simplest crystal structure and absence oftoxic arsenic in comparison with iron arsenides, iron chalcogenide superconductorgenerally attracts greater attention from researchers. In addition, it is relatively easy toprepare iron chalcogenides and it also demonstrates higher safety in application.Meanwhile, iron chalcogenides is reckoned to be ideal model system to understandthe the details of the formation mechanism of superconductivity, which will providetheoretical basics for the understanding of other iron-arsenides superconductors.This study takes iron chalcogenides superconducting materials as the subject.FeSe and Fe(Se, Te) polycrystaline massive materials were prepared by mechanicalmilling and two-step low-temperature synthesis methods. The structure,micro-morphology and superconductivity of the materials were studied in detail, andthe relationship between β-FeSe and superconducting properties was also explored.Through the introduction of3d-elecctron-bearing Cr element into FeSe and Fe(Se, Te)systems, the effect of doping concentration on superconducting property was alsoinvestigated.In this study, effects of mechanical milling on the sintering process,microstructure and superconducting property of the polycrystaline FeSe wereexplored. Furthermore, the possibe mechanism of β-FeSe for milled FeSe systemduring sintering was systematically investigated. The results showed that, with theball-milling time increasing, the content of β-FeSe and Tcincreased simultaneously.When the milling time exceeded50h, the superconductivity was suppressed or even disappeared as β-FeSe transformed into-FeSe during the sitering. It was observedthat the successive sintering process for50h-milled Fe–Se system consists ofdifferent stages: crystallization of amorphous Se；FeSe2at solid–solid reaction stage;melting of Se; Fe7Se8at solid-liquid reaction stage. Nevertheless, the transformationsfrom Fe7Se8to β-FeSe and β-FeSe to-FeSe were not detected by differential thermalanalysis (DTA).In order to investigate the relationship between lattice distortion andsuperconductivity, Te-doped FeSe0.5Te0.5polycrystalline material was synthesizedalso by mechanical milling. It was found that the residual stress introduced bymechanical milling hindered the diffusion of Te in β-FeSe lattice and therefore, thelattice parameter c and d spacing with the prolonging of milling time. In addition, dueto the activity of rectants of Fe and Se, β-FeSe was prone to transformed into-FeSeduring the sitering process after20h milling time, resulting in the degradation ordisappearance of superconductivity.Considering the effect of residual stress introduced by ball milling process,FeSe0.5Te0.5massive material was prepared by two-stage low-temperature synthesisand the influence of sintering temperature on superconductiveiy was studied. It wasrevealed that the superconducting transition temperature of the sample synthesized at550°C reached15.5K, which is a very high value compared to previous massivesamples prepared at ambient pressured through high temperature solid-state sinteringmethod. The two-dimensional nucleation was considered to dominate thelayer-by-layer formation of the lamellar β-FeSe crystal. As a result of the introductionof Te, β-FeSe grew preferentially along crystal plane (002). As stepped alignmentdeviates from the the [100] or [010] direction direction but along the diagonaldirection, which led to the presence of crystal kink on steps for β-FeSe.The effect of Cr-doped concentration on superconducitivity was investigated indetail. It was found that sintering process on phase formation was affected slightly bytiny Cr doping; however, Cr-doping was found to cause lattice distortion for β-FeSe.As the content (x) of Cr for FeCrxSe increased, the superconductivity of the systemwas increased to some extent by the effect of slight negative chemical pressure causedby Cr-doping. Besides, it was suggested that the amount of doping Cr not exceed5at.%, otherwise the superconducting property would be suppressed. Subsequently,FeCr0.05Se sample was synthesized at various low temperatures. It was found that themost suitable sintering temperature of Cr-doped FeSe was450°C, at which the superconducting transition onset temperature, Tconset~10.2K was acquired. As for bulkFeSe0.5Te0.5samples, Cr doping changed the Fermi surface and thus suppressed thesuperconducting property; however, the Tconsetof Cr-doped FeSe0.5Te0.5Cr sample waslower than that of the original FeSe0.5Te0.5sample. Recovery of superconductivityoccurred as doping electron density increased; nerveless, the superconductingproperty began to degrade when the density of such charge carriers reachedsupersaturation.