Effect Of Rare Earth And Oxygen Vacancy Doping On The Structure And Physical Properties Of CeOBiS₂

BiS₂-based materials have attracted much attention,because the similar layered structure with the high temperature superconducting materials of iron based compounds and copper oxide,and the research focused on using element doped or applied pressure to improve its superconductivity.Recently,elements doping on the structure,magnetic and electrical effects of the compound system had reported more,but there were not enough research for the parent phase.In order to study the system of parent phase,we select Ce OBiS₂ as the research subject,exploring the process of crystalline structure of the material,the influence of the magnetic and electric property by controlling the oxygen content,and also doping rare earth elements La,Nd and kin element Y at the site of Ce,exploring two different samples from different doping methods.Specific research is as follows:Polycrystalline samples of CeO_1-xBiS₂(x=0.05,0.08,0.1,0.3,0.5,0.6,0.8)were prepared by solid-state reaction method.Firstly,the phase formation results of the parent phase Ce OBiS₂ samples obtained at different sintering temperatures were analyzed,and the optimal sintering temperature was 800?.On this basis,high pressure annealing was performed on the parent phase samples,and it was found that Bi₂S₃ phase was easily decomposed when annealed at 13 MPa.Therefore,the sintering conditions of oxygen vacancy doped sample Ce O_1-xBiS₂(x=0.05,0.08,0.1,0.3,0.5,0.6,0.8)were selected at 800?at atmospheric pressure.When the oxygen vacancy doping concentration is lower than 0.1,the lattice parameters of the sample decrease with the increase of oxygen vacancy content.When the oxygen vacancy content continues to increase,the lattice parameters of the sample increase with the increase of the oxygen vacancy content and reach the doping limit of the solid solution at x=0.3.The magnetic test results of Ce O_1-xBiS₂ series samples show that the magnetic susceptibility of the samples decreases at low temperature due to the addition of oxygen vacancy.The electrical test results of Ce O_1-xBiS₂ series samples show that the electrical resistivity is obviously reduced in the samples with higher doping content.Polycrystalline samples of Ce in CeOBiS₂ were prepared by solid state reaction method.The polycrystalline samples of Ce in Ce OBiS₂ were partially replaced by Y and La and Nd.Firstly,Y_yCe_1-yOBiS₂(y=0,0.05,0.1,0.2,0.3)series samples were prepared by doping Y elements of the same family with different concentrations and sintering conditions at 800?under normal pressure.When the doping concentration of Y element is less than 0.5,the changes of lattice parameters are irregular.When the doping concentration of Y element is greater than 0.1,the lattice parameter c decreases with the increase of the doping concentration of Y element.Based on the above results,the appropriate doping concentrations of 0.1 and 0.2 elements were selected to prepare La_0.1Ce_0.9OBiS₂ and La_0.2Ce_0.8OBiS₂,as well as Nd_0.1Ce_0.9OBiS₂ and Nd_0.2Ce_0.8OBiS₂.For these four samples,the crystal lattice of the doped sample expands along the c axis compared with the parent phase.The results of electrical property tests show that there is a semiconductor-metal transition in all the Ce-doped samples,and the transition temperature is different due to the slight difference in the electronic structure of different ions.The energy band structure and mechanical properties of Ce O_0.9BiS₂ with oxygen vacancy doping concentration of 0.1 were calculated by first principles.The results of energy band and state density show that the conduction band of the sample passes through the Fermi surface,and the 4f electrons of Ce contribute the most at the Fermi surface,and there is an energy difference of about 0.98 e V between the highest valence band and the lowest conduction band,which indicates that the sample belongs to a semiconductor with a brief energy gap.The mechanical properties of the sample were calculated by using the elastic constants.The results show that the sample has good ductility and the Debye temperature is 191.9 K.