Electron Diffraction And Electrical Conductivity Studies Of Oxygen Ordering, Mobility And Superstructures In YBaCo₄O_7+δ

Since its discovery by Martin Valldor group in2002, YBaCo₄O_7+δhas receivedincreasing attention in the fields of magnetic materials, thermoelectric materials, and oxygenpermeation materials, and so on, due to its special magnetic properties, geometrical frustrationbehavior, structural phase transitons, oxygen adsorption-desorption capability and highchemical flexibility. In particular, it has potential applications in the field of energy, such asoxygen sensors, oxygen separation membranes, and partial oxidation of methane to syngasand solid-oxide fuel cells. However, YBaCo₄O_7+δand its various derivatives have not beenpractially applied in the engineering sciences because of crystallographic challenges related tostructural transitions, geometrical frustration, and oxygen adsorption-desorption properties,and magnetic and electric mechamism, et al. Therefore, the present dissertation carried out astudy about the ordering and mobility of extra oxygen in YBaCo₄O_8.5and its electricproperties. The details are as following:（1） Crystal structure of YBaCo₄O_7+δ（δ=1.5） sample synthesized by a solid-state reactionwas investigated by the electron diffraction. During the observation of Transmission ElectronMicroscopy （TEM）, three different types of electron diffraction pattern were observed, whichindicates that the surplus of oxygen formed several types of ordering or structuralmodulations in the parent lattice of YBaCo₄O₇as the oxygen content varies.（2） Under intense electron-beam irradiation （¹50A/cm2） in a transmission electronmicroscope, the evolution of the electron diffraction pattern was investigated in-situ. Wefound that the extra oxygen atoms could recurrently migrate back and forth in the lattice tocause reversible structural modulation transitions. The oxygen migration can be mainlyattributed to the heating effect of electron-beam. During the electron diffraction, the energycan be transferred from the high incident energy and end up as heat within the specimen,leading to a local temperature increment of up to a few hundred degrees Celsius. Suchtemperature ranges was favorable for absorption and migration of oxygen in such materials.Thus, the reversible evolution of the electron diffraction pattern is observed. When the localtemperature exceeds the oxygen-keeping limit （>400℃） as a result of the accumulated heat inthe sample, and thus, the system sharply releases its extra oxygen, eventually making thesatellite reflections in the electron diffraction disappear. Such reversible evolution of theelectron diffraction pattern suggests the reversible oxygen mobility in YBaCo₄O_8.5.（3） Two kinds of superlattice reflections from oxygenated YBaCo₄O_7+δphase areditermined based on the electron diffraction in the light of the method of the Niggli reducedcell. Corresponding superstructures are determined to be a hexagonal lattice with parametersof aA=10.90（4） and cA=10.02（6） and an orthorhombic lattice with parameters ofaB=18.90（4）, bB=10.90（5） and cB=10.05（0）, respectively. With respect to their respectiveparent hexagonal structure, the two superstructures are determined to be3×3×1structure and3×3×1structure. In addition, the origin of these superlattice reflections and corresponding superstructures is discussed, which is believed to be a result of the orderedarrangement of excess oxygen in the lattice of YBaCo₄O_7+δ. The octahedra centers under theKagomé Co2triangles in the （210） planes are the most possible locations to accommodatethe extra oxygen.（4） The dependence of electric resistivity in YBaCo₄O_7+δceramic on temperature isinvestigated by the Van der Pauw method with DC four point probes and the conductionmechanism is also discussed in the different temperature range. The resistivities ofYBaCo₄O_7+δsamples decline with increasing temperatures, which is an indication ofsemi-conducting properties. In the range of80-300K，the3D-VRH model is used to explainthe conduction process. The small polaron hopping conduction theory is better to describethis conduction mechanism in the range of300-1073K and the value of conductivityactivation energy is calculated to be0.192eV, which is in agreement with the band gap （0.2eV）of YBaCo₄O₇reported by Karppinen.