Study On Preparation And Doping Of 11 System Iron-based Superconducting Materials

The most representative material in the"11"system is the FeSe superconducting material,which is also the simplest-structure superconducting material at present.The lattice structure of FeSe has the tetragonal lattice and the superconducting critical transition temperature is about 8K.Because of the complexity of FeSe phase diagram,the FeSe with tetragonal phase only appears in a narrow range of components,and there is a phase transition from hexagonal phase to tetragonal phase in the system,so it is difficult to obtain single phase FeSe.At present,there are many methods to prepare FeSe bulk materials.Although these methods can be used to prepare superconducting samples,the preparation period is long,the preparation process is tedious,and the samples usually contain heterophase.Recent studies have shown that?-FeSe superconductors can be prepared by solvothermal method.It uses non-toxic and cheap soluble selenium and iron as the starting materials and has the advantages of simple preparation process,shorter preparation period and higher purity of the prepared samples.More importantly,at present,only pure phase FeSe superconducting materials have been prepared by solvothermal method.Therefore,the FeSe superconductor was firstly prepared by solvothermal method.Then the doping study was carried out.The main research work of this paper is as follows:?1?FeSe,FeSe_0.9Te_0.1,FeSe_0.8Te_0.2.2 samples were successfully prepared by solvothermal method.The results of XRD showed that the purity of the prepared samples was very high,and the micro-structure of the samples showed lamellar two-dimensional quadrilateral nano-flake morphology,and its distribution was quite uniform.The PPMS physical properties of the samples were measured.The results show that the superconducting transition temperature of FeSe is 9.3K,which indicates that the samples have excellent superconducting properties and the doping of Te improves the superconducting properties of FeSe.In order to study the surface valence state of the sample,the sample was analyzed by XPS.The results show that the binding energy of Fe2p peak value is higher than the corresponding binding energy in the relevant literature,which indicates that there are a few positive trivalences in the valence state of iron element on the surface of the sample.It is also consistent with the results of magnetic measurements,while the XPS results of Se3d and Te3d show that the valence states of selenium and tellurium are both negative bivalent.Oxygen has a great influence on iron-based superconducting materials,so the properties of oxidized samples are further studied in this paper.The results of PPMS measurement show that the samples will lose their superconducting properties when exposed to air.The further study by XAS shows that the Fe-O interaction of the samples exposed to air is very strong,and the iron elements in the samples mainly exist in the form of positive trivalent,which indicates that the samples must be stored in an oxygen-free environment.?2?The solid-phase reaction method was used to successfully prepare the iron-based superconducting sample FeSe,FeSe_0.5Te_0.5,FeSe_0.4Te_0.6.The XRD patterns show that the samples have good tetragonal lattice characteristics,and the grain micro-morphology with different particle size can be observed from the SEM image.The superconducting transition temperatures of the samples are 9.3K,15K and 13.9K respectively by PPMS measurements.This indicates that the superconducting transition temperature increases with the increase of the doping amount of Te,but when the doping concentration of Te exceeds 0.5,Its superconducting properties will decline.The X-ray photoelectron spectroscopy was used to analyze the surface of the samples.The results show that there are Fe³⁺on the surface of the samples,and the doping of Te does not cause the shift of the peak of Fe spectrum,which indicates that the valence state of Fe does not change after the doping of Te.