Preparation And Characterization Of Co-based Double Perovskite Oxides Sr₂Fe_1-xMo_xCoO₆

The studies of double perovskite oxides （general formula A2B′B″O6） started in1950’s. In the recent decades, double perovskite transition metal oxides have playedan important role in oxide materials research and also have attracted a lot of interest inthe research field, because of the discovery of the copper oxide (Sr₂YRu_1-xCu_xO₆)with high-temperature superconductivity and the molybdenum oxide （Sr2FeMoO6）with colossal magnetoresistance. In this thesis, we have systemically investigated thecrystal structure, electrical and magnetic properties of Co-based double perovskiteoxides Sr₂Fe_1-xMo_xCoO₆（x=0,0.2,0.4,0.5,0.6,0.8,1.0） by a variety ofexperimental methods.Firstly, by using the sol-gel method, we have successfully preparedSr₂Fe_1-xMo_xCoO₆（x=0,0.2,0.4,0.5,0.6,0.8,1.0）. And for the first time by usinghigh temperature and high pressure （HTHP） method, we have prepared the pure phaseof Sr₂MoCoO₆with much higher conductivity as compared to the sample synthesizedby the sol-gel method.By using X-ray photoelectron spectroscopy （XPS）, we have examined thedistribution of the valence states of Fe and Co atoms, and it showed that most of Feand Co atoms exhibited the states of Fe³⁺and Co⁴⁺and the mixed valence states ofFe³⁺/Fe⁴⁺and Co³⁺/Co⁴⁺randomly occupied the B and B sites in Sr₂Fe_1-xMoO₆.The X-ray diffraction patterns of all samples have been refined by GSASsoftware based on the Rietveld method. The phase of parent compound Sr₂Fe_1-xMoO₆was cubic with space group Fm-3m, while the phase of Sr₂MoCoO₆prepared by bothsol-gel method and HTHP method was tetragonal with space group I4/m. With theinter-doping of Fe and Mo atoms, Sr₂Fe_1-xMo_xCoO₆（x=0,0.2,0.4,0.5,0.6,0.8,1.0）showed the structural phase transition from cubic to tetragonal at the critical point of x=0.5.The structural stability of Sr₂Fe_1-xMoO₆has been investigated by in-situ high pressure synchrotron radiation X-ray powder diffraction at pressures up to31GPa,and the equation of state （EOS） has been obtained with no sign of any structuralchanges.During the measurements of the electrical properties, Sr₂Fe_1-xMo_xCoO₆with x≤0.8showed typical semiconducting behavior that can be well described by Mott’s lawfor variable-range hopping conduction. With the further increase of doping, theresistivity of Sr₂Fe_1-xMo_xCoO₆largely increased. For Sr₂MoCoO₆, the resistance wasalmost beyond the range of the measuring instrument （above108ohms at roomtemperature）. However, the resistivity of Sr₂MoCoO₆as prepared by HTHP methodexhibited much higher conductivity but still having semiconducting behavior.For their magnetic properties, we have found that with the increasing amount ofdoped Mo, the magnetism of Sr₂Fe_1-xMo_xCoO₆became weaker. In the low-dopedregion, the samples showed the competition between the ferromagnetic andantiferromagnetic phase. With the increase of x, a decrease of critical temperature Tctwas found. In addition, a transforms from ferromagnetic phase to paramagnetic phasewas observed at x=0.5in Sr₂Fe_1-xMo_xCoO₆. As x further increased, theantiferromagnetic interaction gradually dominated. In the high-doped region, thesamples were antiferromagnetic and the Neel temperature TNmoved toward lowertemperature with the increasing doping amount.