The exploration of double perovskites has led to the introduction of a broad range of high temperature proton conductors with varying conduction properties. To further investigate these materials, acceptor doped Ca2AlNbO6 (CANO) and Ca2YNbO6 (CYNO) have been chosen in order to examine how the choice of B site cations in A2B′B″O6 double perovskite oxides affect the thermodynamics of hydration.
Ca2AlNb1-xTixO6 (x = 0.05, 0.1, 0.15, 0.20) and Ca2YNb0.80Ti0.20O6 were synthesized with the solid state reaction method, and the sintering behavior of Ca2AlNb0.80Ti0.20O6 was investigated by dilatometry. Pulsed laser deposition was used to prepare thin-films of Ca2AlNb0.80Ti0.20O6 deposited on c plane cut sapphire, fused silica, crystalline quartz, and sapphire with TiO2 between as a buffer layer. The microstructure and phase composition of the different samples were studied by scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction. The electrical properties of the materials were studied by AC impedance measurements in the temperature range of 200 °C to 1000 °C in controlled atmosphere. The total conductivity was measured as a function of temperature, oxygen partial pressure, and water vapor partial pressure. Thermogravimetric measurements were performed on Ca2AlNb0.80Ti0.20O6 and Ca2YNb0.80Ti0.20O6 in order to investigate the water uptake of the two samples.
From the total conductivity measurements, the compounds were observed to exhibit mixed ionic and electronic conductivity; n- and p-type electronic conduction predominate at high temperatures under reducing and oxidizing conditions, respectively. A protonic contribution was observed below ~600 °C under wet conditions for Ca2AlNb0.80Ti0.20O6 and Ca2YNb0.80Ti0.20O6. On the basis of the electrical measurements, a defect structure with protons and oxygen vacancies compensating the acceptor dopants was ascribed to the materials.
The temperature and water vapor partial pressure dependencies were modeled based on the simplified defect structure, resulting in values for the hydration thermodynamics and transport parameters. The proton transport is limited by relatively high activation energies of mobility; 0.81 eV and 0.91 eV for Ca2AlNb0.80Ti0.20O6 and Ca2YNb0.80Ti0.20O6, respectively. The high activation energy is believed to reflect protons associated to effectively charged defects formed by site exchange among the B-site cations. Consequenctly, the maximum proton conductivity of Ca2AlNb0.80Ti0.20O6 and Ca2YNb0.80Ti0.20O6 is in the order of 10-6 S/cm.
The hydration enthalpy was estimated to -52 ± 2 kJ/mol and -89 ± 6 kJ/mol for Ca2AlNb0.80Ti0.20O6 and Ca2YNb0.80Ti0.20O6, respectively. At the present time, there is no general consensus on which role the electronegativity of the B-site cations play for the hydration enthalpy. However, Norby has previously suggested a correlation between the hydration enthalpy and the electronegativity difference of the A- and B-site cations in the case of single perovskites. In this thesis, possible generalizations of the correlation to doped double perovskites are suggested.