The Investigations Of Electrical, Magnetic Properties And In-situ High Pressure Studies In Perovskite-type And Double Perovskite-type Oxides

In recent years, because of the discovery of the copper oxide (Sr2YRu1-xCuxO6)with high-temperature superconductivity and manganese-based perovskite oxides(AMnO3) with colossal magnetoresistance, the studies of perovskite (general formulaREBO3) and double perovskite oxides (general formula A2B′B″O6) have attractedmore attentions due to their unique physical and chemical properties. They are widelyused in the areas of catalysts, sensors, fuel cells, magneto-optical materials, et al.In this thesis, we have systemically investigated the crystal structure, electricaland magnetic properties of rare earth perovskites REBO3(B=Fe, V, Al) and Sb-baseddouble perovskite oxides Sr2FeSbO6, Sr2MnSbO6and Ba2FeSbO6by a variety ofexperimental methods.Firstly, we have successfully synthesized REVO3(RE=La, Y) using highpressure and high temperature technique and studied the high-pressure structuralbehaviors of REBO3(B=Fe, V, Al) using angle-dispersive synchrotron X-ray powderdiffraction at room temperature. For REFeO3(RE=Sm, Y), YFeO3undergoes afirst-order isostructural phase transition around26GPa maybe resulting from a highto low spin crossover of Fe3+; the crystal structure of SmFeO3remains stable up to thehighest pressure and the larger compressibility of the FeO6octahedra suggests theevolution of the orthorhombic structure towards higher symmetry configuration athigh pressures. For REAlO3(RE=Sm, Nd), SmAlO3undergoes the phase transitionfrom the orthorhombic (Pnma) structure to the rhombohedral (R-3c) structure at10.4GPa; NdAlO3undergoes the phase transition from the rhombohedral (R-3c) structureto the cubic (Fm-3m) structure at23.6GPa; both of them are caused by the less tiltingand distortion of the AlO6octahedra under high pressures. For REEVO3(RE=La, Y),LaVO3undergoes the phase transition from an orthorhombic (Pnma) structure to a rhombohedral (R-3c) structure at8.0GPa like SmAlO3; however, the structure ofYVO3is much stable throughout the whole pressure range caused by the irregularchanges in the distortion of the VO6octahedra.Secondly, the structural and magnetic properties of REFeO3(RE=Sm, Nd) withB site substitution of non-magnetic atom Al have been investigated. For Al dopingSmAlO3: SmFe(1-x)AlxO3remains the orthorhombic structure within the whole dopingrange and the octahedral tilting distortions of FeO6are found to release while thetolerance factor increases; however, the relationship between the lattice parametersand Al concentration is observed to deviate from Vegard’s rule, and it may be causedby magnetostriction effects; for values in a range0x0.6, the ferromagnetism,antiferromagnetism, and paramagnetism are observed to occur continuously.Moreover, the magnetization and the spin reorientation temperature (Tk) decreasemonotonically as Al content value increases, which are caused by the additionalexchange field Hmvthat will reduce the Fe3+-Sm3+interation. For Al doping NdAlO3:A structural phase transition from the orthorhombic (Pnma) phase to therhombohedral (R-3c) phase occurs as the doped Al is around0.4, at which is alsoaccompanied by the magnetic transition from ferromagnetic to paramagnetic state.Thirdly, we first use high pressure and high temperature (HPHT) method toprepare the double perovskites Ba2FeSbO6and obtain a new type structure ascubic-BFSO. Comparing with the hex-BFSO, the newly discovered cubic-BFSO hasfour times higher Neel transition temperature (TN) at ambient pressure, which may beresulted from the increase of Fe-O-Fe superexchange caused by the lower distortionof FeO6in cubic-BFSO. Besides, during the measurements of the electrical properties,Sr2MnSbO6shows typical semiconducting behavior that can be well described byMott’s law for variable-range hopping conduction.At last, we have studied the high-pressure structural behaviors of Sr2FeSbO6andBa2FeSbO6using angle-dispersive synchrotron X-ray powder diffraction at roomtemperature. Sr2FeSbO6undergoes the phase transition from a cubic (Fm-3m)structure to a tetragonal (I4-m) structure at7.3GPa accompanied by the octahedrarotation along c-axis; however, the structures of cubic-BFSO and hex-BFSO are much stable throughout the whole pressure range. It also indicates that the cubic-BFSO isformatted directly by the sol-gel precursor under high temperature and high pressureconditions instead of the structural transition from hex-BFSO under high pressure.