Defects and transport in fluorite related materials Nd2Ce2O7 and Gd2Ce2O7

In this master thesis, defects and transport properties in fluorite related materials Ln2Ce2O7 (Ln=Nd, Gd) were investigated by electrical measurements and thermogravimetry. Nd2-xCaxCe2O7-δ (x=0, 0.05, 0.1, 0.2) and Gd2-xCaxCe2O7-δ (x=0, 0.05, 0.1, 0.2) were synthesized by solid state reactions and wet chemical methods. Crystal structures were characterized by XRD and the ratio of the chemical elements and microstructure were studied by SEM-EDS. Rietveld refinement was used to determine the cell parameters and occupancy of oxygen ions. The electrical characterization was done by measuring the AC conductivity as a function of temperature from 1100°C to 300°C, of oxygen partial pressure (pO2) and water vapor partial pressure (pH2O). Electrical measurements were also carried out from 100°C to -60°C. The relative mass change of the materials as a consequence of the temperature decrease from 1100°C to 300°C was studied by thermogravimetry under wet and dry conditions. \nNd2Ce2O7 and Gd2Ce2O7 crystallize in fluorite type structure and cubic rare earth oxide structure, respectively. According to the SEM characterization and Rietveld refinement, the solubility of calcium in Nd2Ce2O7 was less than 10% but more than 20% in Gd2Ce2O7. It was observed from curve fitting of pO2 and pH2O dependency of conductivity that oxygen ion conduction predominates from 1100°C to 600°C in wet O2 or Ar and protons become significant below 500°C. It is suggested that association between oxygen vacancies and acceptor dopants happened in doped samples. Oxygen vacancies in Nd2Ce2O7 and Gd2Ce2O7 are not fully hydrated at 350°C which is in agreement with the net mass difference in TG results measured under wet and dry conditions. Hydration enthalpy cannot be obtained by modeling results of pH2O and pO2 dependency of conductivity but it should be fairly low because of the low amount of water absorption from 1100°C to 300°C. Proton mobility and dissolution decrease with decreasing ionic radius of lanthanides in Ln2Ce2O7. The decreasing proton mobility can be due to the decreasing polarization of lattice or symmetry. The enthalpy of oxygen ion mobility in Ln2B2O7 (Ln=lanthanides, B=Ce, Zr) decreases with increasing radius of Ln3+ due to larger unit cell volume facilitating oxygen ion mobility. Also, the enthalpy of oxygen ion mobility in Ln2Ce2O7 with a high degree of anion disorder is much higher than that in Ln2Zr2O7 with ordered oxygen ions. It is suggested the different oxygen ion migration pathways in Ln2B2O7 with large difference in anion disorder. The higher cation disorder in Ln2Ce2O7 than Ln2Zr2O7 can also affect oxygen ion mobility due to the trapping effect between cation antitsite and oxygen vacancies. \nAt room temperature, the conductivities of Nd2Ce2O7, Gd2Ce2O7 and Gd1.95Ca0.05Ce2O7-δ are around 1×10-5 Scm-1. TG measurement and the relation of σ∝(pH_2 O)^2.8 indicate the enhanced conductivity should be ascribed to protonic conduction. It is suggested that protonic conduction at room temperature is independent of grain size and doping concentration. The conductivity change at 0°C suggests a phase transition from water to ice. Protons are thereby expected to transport in water condensed in micro-sized pores or cracks in the form of hydronium ions.