Synthesis And Properties Study Of New Intercalation Compounds Of FeSe

Iron based superconductor is regarded as the second genuine family of unconventional high temperature superconductors subsequently to copper oxide superconductor,and attract much attention since its discovery.FeSe composing of a stack of edge-sharing FeSe4-tetrahedra layers along the c-axis,has the simplest composition and structure within the iron-based superconductor,which is considered as the prototype of the whole Fe-based superconductor family.The Tc of FeSe itself is only 8K,but it could be greatly improved by applying external pressure,doping,interface engineering and the intercalation of the guest.Therefore,the research on the FeSe system has been worldwide attracted the continuous interest.In recent years,considerable progress has been brought in layered iron-based superconducting chalcogenides intercalated with molecular guests.The intercalation of molecular guests solve the problems related to phase separation associated to AxFe2-ySe materials growing at high temperature,and Tc of the FeSe system has been increased up to 46K.Because of intriguing chemistry and adjustable electric and magnetic properties,these novel materials have been intensively studied,especially in the field of solid state chemistry and condensed matter physics.Despite the instability and the presence of magnetic impurity phases in the product,the intercalates have at least two indisputable advantages:first,both the chemical nature and orientation of organic molecules between the FeSe layers could modify the structure resulting fine tuning of superconductivity.Second,this system is very flexible.A variety of molecular species compatible with alkali metals,as well as the doping in the host structure(either on Fe or Se sites),gives a vast array of new materials,and it greatly enriches the family of FeSe-based superconductor.In this dissertation,we aim to investigate the synthesis and properties study of new FeSe-based superconducting materials by changing the precursors and experimental methods,on condition that ethylenediamineis is taken as the intercalation guest.Specific research content mainly consists of three chapters:1.we successfully synthesized a new FeSe-based superconductor(C2H8N2)0.3Fe1.04Se by the solvothermal method,demonstrating that the EDA molecule alone can be intercalated into FeSe.Rietveld refinement revealed the leaning of molecular plane of monolayer EDA between the FeSe layers,which resulted the minute difference between the lattice parameters a and b,in other words,the structure of the host FeSe underwent a structural transition from tetragonal to orthorhombic in the intercalation process.Magnetization measurement revealed two superconducting transitions:the superconducting transition at 30K may be attributed to the measly impurity phase,and the superconducting transition at 10K was assigned to the(C2H8N2)yFeSe after excluding FeSe in final product.The intercalation of only EDA into FeSe significantly expanded the interlayer spacing,but slightly improved Tc.The further filling of Li ions into(C2H8N2)yFeSe by reacting with n-butyl lithium could formed Lix(C2H8N2)yFeSe,and improved Tc to 45K.The crystal structure of Lix(C2H8N2)yFeSe was returned to the tetrahedral symmetry.Therefore,the electron doping is the effective factor to enhance Tc of FeSe-based compound.2.We synthesized a series of(C2H8N2)yFeSe1-xSx(0?x?0.3)via solvothermal method.With the substitution of S for Se,the symmetry of the substituted derivatives was consistent,but all off the lattice parameters along the a,b and c axis were compressed due to the smaller ionic size of S relative to that of Se.Magnetization measurement revealed that the Tc was gradually suppressed as S content increases,and eventually superconductivity vanishes at x=0.2,suggesting the doping of S is unfavorable to superconductivity.The superconductivity of FeSe1-xSx(x=0.2 and 0.3)was completely suppressed by the intercalation of ethylenediamine,which indicated that the doping of S made the superconductivity of the precursor more vulnerable to the intercalation of ethylenediamine.3.We successfully synthesized an orthorhombic phase of Lix(C2H8N2)yFe2Se2 with Tc?30-35K by amine exchange method,which was different from the tetrahedral phase Lix(C2H8N2)yFe2Se2 with Tc=45K synthesized by solvothermal method.In the orthorhombic phase,the increase of lithium ion content does not changed the structure symmetry but slightly improved Tc.However,Tc of orthorhombic phase was lower than that of tetrahedral phase,which was not impute to insufficient lithium ion content.Compared with the precursor Lix(NH3)yFe2Se2,the Lix(C2H8N2)yFe2Se2 underwent a structure transition from tetrahedral to orthorhombic during the amine exchange reaction accompanying by the decrease of Tc,indicating that Tc was influenced by structure.In addition,a new phase Lix(C3H10N2)yFe2Se2 with primitive tetrahedral symmetry was obtained by substituting 1,3-diaminopropane for ammonia molecules in Lix(NH3)yFe2Se2.Nax(C2H8N2)yFe2Se2 and Nax(C3H10N2)yFe2Se2 were synthesized by the same method above-mentioned,which were similar to the compounds synthesized by sonochemical method or solvothermal method proved by both the XRD and magnetic data.After the substitution of ethylenediamine for ammonia in Nax(NH3)yFe2Se2,the structural symmetr and the Tc at 45K of Nax(C2H8N2)yFe2Se2 were retained,but the interlayer spacing significantly increased.However,the substitution of 1,3-diaminopropane for ammonia in Nax(NH3)yFe2Se2 resulted in a structure transition from tetrahedral to orthorhombic,although Tc did not change significantly.In conclusion,the influence of substitution with organic amine on structure and Tc of Ax(NH3)yFe2Se2(A=Li,Na)is related to the species of organic amine and alkali metal.The Tc of multiple superconducting phases was related to the interlayer spacing dependence(d),i.e.,a higher d resulted in a higher Tc.