| Owing to the abundant crystal structures and novel physical properties,layered transition metal compounds are always the hot topic in the field of condensed matter physics and material chemistry,which contain both the cuprate superconductors and the iron-based superconductors.The cuprate superconductors adopt a layered perovskite structure and consist of two alternately stacking substructures: the Cu O2 active block and the charge reservoir block.The iron-based superconductors also have quasi-two-dimensional layered structures and comprise anti-fluorite type Fe Se/Fe As layers.In addition,some promising thermoelectric materials,such as Bi Cu Se O,La Cu Se O,Bi2YO4Cu2Se2,etc,also belong to the layered transition metal compounds.Consequently,exploring new layered transition metal compounds will be helpful for the discovery of new high-temperature superconductors,high-performance thermoelectric materials and compounds with novel properties.In this thesis,we introduce the single crystal growth methods and properties of two layered transition metal compounds: Ba2 Cu O2Cu2Se2 and BiNi2 Se.Polycrystal sample of Ba2 Cu O2Cu2Se2 has been synthesized under high-pressure and hightemperature conditions before.It crystallizes into space group I4/mmm(No.139)and has a layered structure,which is composed by alternately stacking two distinct layers of a Cu O2 layer and a anti-fluorite type Cu Se layer along the c-axis with Ba sheet intercalating between them.While BiNi2 Se is a new layered ternary compound.The thesis contains the following five chapters:In the first chapter,we firstly give an outline of the theory of superconductivity and several examples of typical superconductors,including cuprate superconductors,iron-based superconductors and Bi S2-based superconductors.In addition,we introduce the synthesis method of Ba2 Cu O2Cu2Se2 under high-pressure and high-temperature conditions,and we also give a table of compounds that have the same structure with it.Finally,we introduce several transition metal compounds.In the second chapter,we give a brief overview of modified Bridgman method and characterization techniques that include X-ray diffraction,Scanning Electron Microscope measurement,electrical transport properties measurement,and magnetic susceptibility measurement.In the third chapter,we report the single crystal growth method,crystal structure and physical properties of Ba2 Cu O2Cu2Se2.Single crystals of Ba2 Cu O2Cu2Se2 were successfully grown using a modified Bridgman method under ambient pressure.The powder X-ray diffraction pattern is in consistent with previous report.The result of single crystal X-ray diffraction analysis shows that there are vacancies both on the Cu sites and O sites in Cu O2 layers.And it also shows that the crystal parameters are slightly larger than the high-pressure counterparts.The electrical resistivity measurement shows that Ba2 Cu O2Cu2Se2 is a semiconductor with a band gap evaluated to be 139 me V.The temperature dependence of magnetic susceptibility suggests a paramagnetic behavior in the measured temperature range.In the fourth chapter,we report the single crystal growth method,crystal structure and physical properties of BiNi2 Se.The X-ray diffraction pattern shows that BiNi2 Se crystallizes in the tetragonal Pmma(No.51)space group and it has an obviously layered structure.The Energy Dispersive X-ray spectrum reveals the existence of vacancies both on the Ni sites and Se sites.The temperature dependence of electrical resistivity suggests a metal behavior and hall coefficient measurement demonstrates that the dominant carriers are hole-type.The magnetic susceptibility measurement reveals that BiNi2 Se has a moderate magnetic anisotropy.In the fifth chapter,we summarize the studies and give a brief prospect of the layered transition metal compounds. |
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