The High Pressure Synthesis and Physical Property Researches of Novel Quasi-one Dimensional Quantum Fuctional Materials

Author:Zhang Jun

Supervisor:jin chang qing wang xian cheng

Database:Doctor

Degree Year:2018

Download:109

Pages:150

Size:8994K

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Due to the lower dimension comparing to two or three dimensional materials,the materials with one dimensional structures exhibit many novel physical properties.In this dissertation,several ternary transition metal chalcogenides with quasi-one dimensional structure have been discoveried and synthesized initially employing a high temperature and high pressure route.The crystal structures,electronic transport behaviors,magnetic properties and thermaldynamic properties have been studied systematically.The main results are shown as follows:(1)Research on thermoelectric materials has received considerable attention because of their potential applications in the field of energy conversion.The tin chalcogenides are reported as thermoelectric candidate materials frequently.The single crystal Ba9Sn3Te15 has been synthesized initially at high temperature and high pressure conditions.Single crystal X-ray diffraction results show that Ba9Sn3Te15crystallizes into a quasi one dimensional crystal structure.The compounds of Ba9Sn3(Te1-xSex)15 with x=0–1 are prepared by substituting Te using Se and all compounds are semiconductors.The band gap and Seebeck coefficient increase as the increasing content of Se.Ab initio calculations were also performed to study the density of states and band structures of Ba9Sn3Te15 and Ba9Sn3Se15,which are well consistent with band structure of one dimensional conductors.It is referred that SnTe6chains behaves semiconducting due to Umklapp scattering.It is critical to synthesize Ba9Sn3(Te1-xSex)15(x=0–1)single crystal and measure the anisotropic Seebeck coefficient for obtaining a high efficient thermoelectric material.(2)The coupling strength between adjacent spin chains in one dimensional materials plays an important role in the magnetic ordering temperature and electronic transport property.The distance between adjacent spin chains in BaVSe3 is 6.999?,which leads to the increased coupling interaction and the electronic structure of BaVSe3 is three dimensional.Ba9V3Se15 has been prepared at high temperature and high pressure conditions.The powder X-ray diffraction and refinement results reveal the crystallographic data and the distance between adjacent spin chains is 9.5745?.The magnetic susceptibility,heat capacity measurements and calculations confirm that Ba9V3Se15 can be regarded as an effective ferromagnetic chains composed of the repeating unit of V(2)(↓)V(1)(↑)V(2)(↓)cluster.(3)Establishing the relationship between the coupling strength between the adjacent spin chains in one dimensional materials and magnetic ordering temperature,electronic transport behavior is benefit for designing one dimensional materials according to different requirements.Ba9V3Te15 with one dimensional ferromagnetic chains has been synthesized initially employing a high temperature and high pressure route.The refined results of powder X-ray diffraction data indicate that the distance between adjacent vanadium chains is 10.1848?.The magnetic susceptibility and neutron diffraction confirm that the ferromagnetic ordering forms at 3.65 K and the moments arrange ferromagneticly along c axil.Comparing of magnetic and electronic behaviors of Ba VSe3,BaVS3,Ba9V3Te15 and Ba9V3Se15,it is found that the magnetic ordering temperature decreases and the electronic transport behaviros changes from metal to insulator as the coupling strength weakens.(4)In iron based superconductors,the iron chalcogenides usually display a larger local magnetic moment,different Fermi surface and abundant phase diagram comparing to the iron pnictides.The Ba9Fe3Se15 single crystal was grown at high temperature and high pressure conditions.The single crystal X-ray diffraction gives explicit crystal structure.The magnetic susceptibility,heat capacity and neutron diffraction measurements prove that the antiferromagnetic ordering forms at 15 K and the moments parallel to ab plane,presenting an incommensurate toroidal arrangement along c axil.Combined of high pressure X-ray measurement,Raman spectrum technique,X-ray emission spectrum and electronic transport measurement,it is found that the crystal structure of Ba9Fe3Se15 keeps stable with the pressure up to 60 GPa and a lattice contraction occurs at approximate 35 GPa,which is accompanied by the emergence of electronic transport behavior transition from insulator to metal and electronic state transiton from high spin to low spin.Further,a possible superconductivity occurs at 6 K.Therefore,Ba9Fe3Se15 is a possible novel iron based superconductor with quasi one dimensional structrue.