Exploration Of New Superconductors And Study On Properties And Structures Of Transition Metal Oxide Functional Systems

This thesis mainly focuses on the exploration of new superconductors and transition metal oxide functional materials,in which emphasis is placed on the Pt-Ge ternary intermetallic compounds and their potential superconductivity.In addition,the synthesis method of rare earth iron oxide Lu_1-xSc_xFe O₃ single crystals and orthogonal o-Lu_1-xSc_xFe O₃ nanowires was also studied.The structures of the synthesized compounds were characterized by X-ray diffraction（XRD）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,X-ray photoelectron spectroscopy（XPS）and other techniques;and the low temperature physical properties were measured by the integrated physical property measurement system（PPMS）and vibrating sample magnetometer（VSM）.The corresponding electronic structures were revealed by the first-principles calculation.Four typical material systems are studied in detail in this thesis:the filled-skutterudite intermetallic compound superconductor PrPt₄Ge₁₂,the half-Heausler intermetallic compound c-HfPtGe,the novel intermetallic oxide superconductor Hf₃Pt₄Ge₂O and the rare earth iron oxide Lu-Sc-Fe-O.Among these,the first three systems are closely related,and the last one is relatively independent.1.Study on the filled skutterudite intermetallic compound superconductor PrPt₄Ge₁₂.Firstly,the synthesis method of PrPt₄Ge₁₂ sample was optimized.The high quality polycrystalline samples were obtained through the combination of arc melting and secondary sintering.Secondly,a series of nominal Pr_1-δPt₄Ge₁₂（δ=0,0.1,1/8,0.2,0.3,0.4,0.5）samples were prepared for exploring the effect of Pr loss on the crystal structure and superconductivity.A special structure with broken inversion symmetry was identified atδ=1/8,and the microscopic origin was attributed to the mechanism of"asymmetric distortion of[Pt Ge₆]octahedron along I₃ axis induced by unilateral Pr vacancy".Thirdly,the Pt Ge₃ skeleton in PrPt₄Ge₁₂ was partially replaced by two elements,i.e.Pr[(Pt_1-xRh)_x(Ge_1-xSb _x)₃]₄.A series of samples were synthesized and the physical properties were characterized.Finally,the nominal Hf Pt₄Ge₁₂ samples were synthesized,in which two diamagnetic signals can be observed in a few samples,indicating the possible existence of superconducting transition in this system.2.Studies on half-Heausler intermetallic compound c-HfPtGe.Firstly,based on the method mentioned in previous literatures,c-HfPtGe polycrystals were successfully synthesized,whose quality was verified by powder X-ray diffraction and electron microscopy.In addition,the influence of synthesis temperature on the phase formation of c-HfPtGe was further explored.Secondly,the low temperature physical properties of c-HfPtGe,including electric transport and magnetic properties,were investigated and compared with o-HfPtGe.Thirdly,the electronic band structure of c-HfPtGe is provided by first principles calculation,which helps to understand the measurement results.Finally,the sample HfPtGe₂ was also successfully synthesized,which didn’t show any signs of superconductivity in physical property measurements.3.Study on novel intermetallic oxide superconductor Hf₃Pt₄Ge₂O.Firstly,based on the statistic cluster analysis,a method which can quickly and effectively identify the elemental composition of the tiny amount of superconducting phase in sample was proposed.The analyzing results indicate that the superconducting phase Hf₃Pt₄Ge₂O contributes to the diamagnetic signal observed in Hf Pt₄Ge₁₂ sample.Secondly,high quality polycrystalline Hf₃Pt₄Ge₂O samples were synthesized by solid state reaction method.The unit cell structure model was constructed based on the results obtained from EDX,TEM,XRD and other characterization techniques.The lattice parameters and atomic coordinates were also extracted from the XRD refinements.Thirdly,the superconductivity of Hf₃Pt₄Ge₂O was revealed by the low temperature electric transport,magnetic property and specific heat measurements.The results inidcate that Hf₃Pt₄Ge₂O is a weakly coupled type-Ⅱsuperconductor with a superconducting transition temperature of about 4.1 K.Fourthly,based on the constructed crystal model,the first-principle calculation was performed,and the band structure and density of states were obtained.Fifthly,the importance of oxygen atoms in maintaining the crystal structure of Hf₃Pt₄Ge₂O is clarified.The effect of oxygen atoms on the structure and physical properties of Hf₃Pt₄Ge₂O_1+δ（δ=-1/4,-1/8,0,+1/8,+1/4,+1/2）was investigated by preparing a series of samples with differentδ.The result suggests that the oxygen atoms play an important role in the formation of Hf-Pt mixed-occupied Hf₃Pt₄Ge₂O₁₊δalloy phase,which is further verified by XPS.Sixthly,chemical doping in Hf₃Pt₄Ge₂O was attempted.Three compounds with nominal composition of(Hf_0.75Sc_0.25)₃Pt₄Ge₂O,Hf₃(Pt_0.75Ir_0.25)₄Ge₂O and Hf₃Pt₄(Ge_0.75Ga_0.25)₂O were synthesized,and their superconducting properties were also investigated.Finally,the Hf₆Pt₁₆Ge₇ and Ge₆Pt₁₆Hf₇ samples were synthesized.The effect of oxygen atoms was further discussed,and Hf₃Pt₄Ge₂O was identified to be a potential electride.4.Study on rare earth iron oxide Lu-Sc-Fe-O.Firstly,on the basis of previous studies,an economical and convenient flux method was developed.By adjusting the compositional proportion of K₂CO₃-B₂O₃-Bi₂O₃ ternary flux,orthogonal,hexagonal and cubic Lu_1-xSc_xFe O₃ single crystals can be obtained.Then,the chemical composition and lattice parameters of the three compounds(o-Lu_0.96Sc_0.04Fe O₃,c-Lu_0.2Sc_0.8Fe O₃ and h-Lu_0.67Sc_0.33Fe O₃)were analyzed by EDX and single crystal X-ray diffraction.The magnetic properties were also measured at low temperatures.Secondly,the microstructure of the hexagonal h-Lu_0.67Sc_0.33Fe O₃ single crystal was studied by XRD,SEM-EDX and TEM.The morphology of the“cloverleaf”ferroelectric domains on the surface was observed by SEM,and the electric polarization revealed by the displacements of rare earth Lu ions in the vicinity of domain walls were observed by the spherical-aberration-corrected TEM.Thirdly,orthogonal o-Lu_0.55Sc_0.45Fe O₃ polycrystalline nanowires were prepared by sol-gel method and electrostatic spinning method.The composition and structure of the samples were studied by SEM-EDX,XRD and TEM,and the magnetic properties were measured at low temperatures.