The Synthesis And Physical Properties Of New Supercond-Uctor (Li_0.8Fe_0.2)OHFeSe

Due to the attractive application prospect, much attention has been paid to superconductivity since its discovery. However, the Tc of the superconductors that already existed is very low, which hinders the further application. In order to pursue superconductors with higher Tc, numerous scientists have paid infinite enthusiasm and perspiration. The discovery of high temperature cuprate superconductors lit a beacon for the exploration of high Tc superconductors. Although the Tc of cuprates could reach up to164K, the mechanism of its high temperature superconductivity is still unclear. Moreover, the cuprates are very difficult to be machined, which restricts its wider application. The discovery of iron-based superconductors is another great breakthrough in the field of high temperature superconductivity. By comparing these two high Tc superconductors scientists hope to discover the mechanism of high temperature superconductivity, which could provide guidance for seeking high temperature superconductors. The FeSe-derived superconductors were believed to be the significant systems to investigate the mechanism of high temperature superconductivity. However, the current FeSe-derived superconductors are not suitable for further physical research because of phase separation or air sensitive. Therefore, it is crucial to develop new FeSe-derived superconductors.This dissertation presents a new FeSe-derived superconductor (Lio.gFeo.2)OHFeSe which is successfully synthesized by means of hydrothermal method, with Tc up to40K. By combination of powder X-ray diffraction(XRD), neutron powder diffraction(NPD) and nuclear magnetic resonance(NMR), the structure of (Lio.sFeo.2)OHFeSe was unambiguously determined. Moreover, the NMR, magnetic susceptibility and specific heat measurement suggests that a canted antiferromagnetic order occurs at low temperature, and coexists with superconductivity. Moreover, we successfully synthesized a series of (Lio.8Feo.2)OHFeSe1-xSx samples through the substitution of S for Se. The superconducting properties, crystal structure and antiferromagnetic properties with different S doping levels were systematic studied and the phase diagram is established. In addition, the relationship between the superconductivity and the structure of FeSe1-xSx layers is also discussed.The dissertation is consisted of three parts as following: 1. Brief overview of iron-based high temperature superconductorsThis chapter briefly introduces the history of superconducting materials in the first place. Then, the structure and transport properties of iron-based superconductors are introduced in detailed. The relationship between structure and superconductivity is also under discussion. After that, the representative phase diagrams of different systems are referred. Finally, we briefly summarized the synthesis methods of iron-based superconductors.2. The synthesis and superconduting properties of new superconductor (Li0.8Fe0.2)OHFeSeThis chapter introduces a newly synthesized iron-based superconductor synthesized by hydrothermal method. Using the combination of powder XRD, NPD, NMR and the results of chemical composition analysis, the new superconductor was determined as (Li0.8Fe0.2)OHFeSe with alternate stacking of (Li0.8Fe0.2)OH layers and FeSe layers along c-axis. The resistance and susceptibility measurements suggest the Tc of (Li0.8Fe0.2)OHFeSe is up to40K, and the magnetization under pressure indicates the inhibiting effect of pressure on superconductivity. Moreover, NMR and susceptibility under1T suggest a magnetic order occurs at～8K. Finally, the specific heat under different magnetic fields confirms that the magnetic order should be canted antiferromagnetic, and coexists with superconductivity.3. Superconductivity and phase diagram of (Li0.8Fe0.2OHFeSe1-xSxIn this chapter, we successfully synthesized a series of (Li0.8Fe0.2)OHFeSe1-xSx samples. The lattice parameters of a and c axes monotonously decrease with S doping. The susceptibility measurements indicate the Tc is gradually suppressed with the substitution of S for Se, and superconductivity vanishes at x=0.90. However, the temperature of the antiferromagnetic transition remains unchanged with the S content, suggesting that antiferromagnetic order should arise from the (Li0.8Fe0.2)OH layer. Finally, the phase diagram is established according to the susceptibility and specific heat data. In addition, through analysis of the relationship between the superconductivity and the structure of FeSe1-xSx layers, we find that the relationship between superconductivity and the structure in FeSe-derived is distinct from that in FeAs-derived superconductors.