The Preparation,structure Under High Pressure And Thermoelectric Properties Investigation Of Several Chalcogenides

High pressure physics is a subject to study the crystal structure,equation of state,the mechanics,optics and electrics of materials under high pressure.High pressure research can bring discovery of new structures and novel properties of materials,which have not appeared at the ambient pressure.Therefore,high pressure is an effective method for synthesizing new materials and optimizing properties of condensed matter.Chalcogenides can be used in many important fields and they also have been drawn much attention due to their plentiful physical properties under high pressure.In this paper,five kinds of chalcogenides of Bi Cu Ch O(Ch = S,Se,Te),La Cu SO and Pb Se2 were prepared by high temperature and high pressure(HPHT)method.The structural stabilities of Bi Cu Se O,Bi Cu SO,La Cu SO and Pb Se2 under high pressure have been investigated systematically by Raman and angle-dispersive X-ray diffraction(ADXRD).At the same time,the thermoelectric properties of Bi Cu Ch O(Ch = S,Se,Te)and Pb Se2 were studied and discussed,some valuable experimental results were obtained.1.Polycrystalline Bi Cu Se O,Bi Cu SO and La Cu SO were prepared by HPHT,and their Raman modes were assigned using the method of factor group analysis.The stability of these chalcogenides under high pressure was systematically studied by ADXRD and Raman spectroscopy.The results are listed as follows: For Bi Cu Se O,Bi Cu SO and La Cu SO,a pressure-induced isostructural phase transition from tetragonal structure to collapsed tetragonal structure occurred at 13.97 GPa,10.41 GPa and 11.64 GPa,respectively.The value of a-axis increased slowly and c-axis drastically decreased with increase of pressure.The phase transition is attributed to its layered structure.The shearing movement of the charge reservoir layer during compression is a possible driving force for the isostructural phase transition.The shearing movement of the charge reservoir layer in the ab plane causes the lager a-axis.Because the weak covalent bond between the layers,so the c-axis is particularly vulnerable to compression under pressure.The high pressure Raman results of Bi Cu Se O and La Cu SO show a new peak at 14.8 GPa,and the slope of pressure dependence of Raman shifts varies with the pressure changed at 10.8 GPa.These are consistent with the results of the ADXRD experiments.2.Bi Cu Se O precursors were prepared by secondary solid phase method,and then bulk polycrystalline materials were prepared by pressureless sintering and high pressure sintering.The results of XRD and density functional theory show that intrinsic defects of Bi Cu Se O can be suppressed by high pressure,therefore,after the high pressure sintering it has higher hall mobility and Seebeck coefficient.Meanwhile,due to grain refinement at high pressure,smaller thermal conductivity is obtained.3.Rietveld refinement was employed to perform structural analysis of Bi Cu Ch O prepared by HPHT.The thermoelectric properties of Bi Cu Ch O were studied.With the increase in the Ch2-ion radius,the conductivity activation energy of Bi Cu Ch O decreases,the Seebeck coefficient and the resistivity decrease,and the thermal conductivity decreases.The maximum figure of merit(ZT)reaches 0.03,0.31 and 0.65 for Bi Cu SO,Bi Cu Se O and Bi Cu Te O,respectively.4.The solid solution of Bi Cu Te-Bi Cu Se O was synthesized by HPHT.Their microstructure and thermal performance are studied.The results show that the sample with rich Se has smaller particle size(< 1 mm),and sample with rich Te exhibits more apparent layer structure.With the increased content of Te,the resistivity and lattice thermal conductivity decrease,and the dimensionless ZT values increase.5.We have synthesized the tetragonal Pb Se2 by the combination of the mechanical alloy and HPHT method.The structural stabilities of Pb Se2 under high pressure have been investigated by ADXRD.The experimental results show that the structural phase transition of Pb Se2 begins from 19.93 GPa,and the triclinic structure is determined by the CALYPSO software.The volume shrinkage is 18.4% across the phase transition.The theoretical calculations show that the structural phase transition is accompanied by semiconducting to metal transition in Pb Se2.6.The thermal performance of Pb Se2 was studied.The results of resistivity,Seebeck coefficient and differential thermal analysis show that tetragonal Pb Se2 decomposes into cubic Pb Se and Se above 400 K.The thermal conductivity of Pb Se2 is much smaller than that of Pb Se synthesized by the same method.The results of the density functional theory calculation show that the very low thermal conductivity of Pb Se2 is due to its weak bonding force and strong non-harmonic chemical bonds between atoms.