The Study Of The Physical Properties Of Transition Metal Diselenides Synthesized By High Pressure And High Temperature

Transition metal dichalcogenides(TMDCs)are characterized by their quasi twodimensional layer structure similar to graphene.TMDCs show broad properties ranging from insulator,metal,superconductivity to charge density wave and topological phase due to their diverse chemical compositions and polymorphisms.TMDCs have eleven polytypes,Ta Se2 shows seven:1T-,2Ha-,3R-,4Ha-,4Hb-,4Hc-,6R-,and five of them belong to Nb Se2(i.e.,1T-,2Ha-,3R-,4H-,4Hd?-and 4Hd?-).Theoretical investigation indicated that temperature and high pressure could induce phase transition between those polytypes accompaned by intriguing physical properties.Those interesting polymorphisms provide a new route to explore multifunctional materials.Nevertheless,the studies of pressure-induced phase transition,electronic structure change,optical properties and magnetic properties have not been investigated deeply.This thesis have selected Ta Se2 and Nb Se2 as the objects of study,we have obtained three polytypes of Ta Se2 and Nb Se2 under high temperature and high pressure.We have characterized samples by combination of the X-ray diffraction,scanning electron microscope(SEM),transmission electron microscopy(TEM),thermogravimetric and differential thermal analysis.The in-situ high pressure X-ray diffraction study of those materials was carried out in DAC,the results are given as follow:(1)We have obtained 2Ha-Ta Se2 and two metastable phase 4Hb-Ta Se2 and 6R-Ta Se2,also 2Ha-Nb Se2.Reaction temperature plays a key role in the sample morphology of 2Ha-Nb Se2 by SEM.(2)The crystal structural stability of 2Ha-Ta Se2 under high pressure was conducted by in-situ high pressure X-ray diffraction study.It shows that the structure was stable up to 38 GPa,and the bulk modulus was B0 = 66(2)GPa by fitting the B-M equation.The compression of the lattice shows a strong anisotropy,and the c-axis is much easier to be compressed than the aaxis.The ratio of axial compressibility(?c/c0)/(?a/a0)rapidly increased from 0 to 3 in the range of 0-3 GPa,then gradually down to 2.5 and stayed stable with higher pressure.That is because the relatively weak Van der Waals forces below 3 GPa.However,the selenium atoms approach to each other at higher pressures,the increased repulsive force between them in adjacent layers significantly increases which lowers the reduction rate of the c-axis of 2Ha-Ta Se2.(3)The crystal structural stability of 4Hb-Ta Se2 under high pressure was conducted by in-situ high pressure X-ray diffraction study and its crystal structure is stable up to the highest pressure 42 Gpa with its bulk modulus B0 = 59(1)GPa.The compression of the lattice shows a strong anisotropy,the c-direction is compressible than the a-direction.The ratio of axial compressibility(?c/c0)/(?a/a0)increases rapidly from 0 to 3.3 over the 0-3 GPa pressure range,then gradually decreased down to 2 with higher pressure.The smaller bulk modolus of 4Hb-Ta Se2 compared with 2Ha-Ta Se2 maybe due to its specific trigonal prismatic and octahedral coordination of metal atoms layer alternative arrangements along c-axis.(4)The crystal structural stability of 2Ha-NbSe2 under high pressure was examined by in-situ high pressure X-ray diffraction study and no structral phase transition was observed up to 31 GPa,and its bulk modulus was B0 = 78(2)GPa.The compression of the lattice shows a strong anisotropy also.The ratio of axial compressibility(?c/c0)/(?a/a0)increased gradually from 0 to 2 over the 0-10 GPa pressure range,then became stable with the higher pressure.