Studies On Ni-S Binary Compounds And Bi_2-xPb_xNi₃Ch₂（Ch=S, Se） System

In this thesis, I gave a brief introduction on research history, classification methods and practical applications of superconductors. Detailed reviews of the chalcogenide superconductors are made. Our research works are concentrated on the preparation, crystal structure and physical properties of binary nickel sulfides and Pb-doped solid solution Bi2-xPbxNi3Ch2(Ch=S, Se). The main results are as following:(1) Several binary nickel sulfides Ni3S2, ε-NiS, δ-NiS, NiS2and Ni9S8were synthesized by solid-state reaction. Ni3S2, ε-NiS and Ni9S8are good metals whereas a metal-semiconductor transition taken place in NiS2near30K, temperature dependence of magnetic susceptibility indicated that Ni9S8follows Curie-Weiss law.(2) Using a novel temperature-controlled hybrid-microwave method, four binary compounds Ni3S2,δ-NiS, ε-NiS and NiS2are successfully synthesized under argon atmosphere. XRD phase analyses indicate that single-phased Ni3S2can be prepared easily, while δ-NiS and ε-NiS can be obtained as the major phase by adjusting the preparing condition. It is difficult to prepare single-phased NiS2sample due to the vaporization of sulfur. Ni3S2shows paramagnetic metal behavior.δ-NiS shows negative thermal expansion property from150K to260K, the metal-nonmetal transition temperature region broadened due to grain refinement.(3) By the solid-state reaction, Pb-doped solid solubility Bi2-xPbxNi3Ch2(Ch=S,Se) were prepared and their physical properties were investigated. The superconductor transition was observed in quenched PbBiNi3S2, but there was no zero resistivity.The solid solubility limit of Pb in annealed PbxBi2.xNi3S2is near1.38, while that of Bi in BixPb2-xNi3S2is about0.20. With the Pb content increasing, the lattice parameters a, c decrease significantly, the lattice parameter b decreases firstly and then increases after x=0.5.The solid solubility limit of Pb in PbxBi2-xNi3Se2is near1.0. The lattice parameters a decrease with Pb content increasing, but b and c increase after Pb-doping, which made the unite cell stretches along c axis and enhance the two-dimensional performance of Bi2Ni3Se2.The samples annealed at775K of the two series are good metallic and a spin-glass behavior was investigated in ZFC condition. The residual resistivity increases with Pb content increasing, it reaches the maximum value when x=1.0. The spin-glass freezing temperature Tf increases with the Pb content increasing.