La2Mo2O9, as a new fast ionic conductor, has been investigated widely due to its high
ionic conductivity which is comparable to those of the commercialized materials. However,
little work has been reported on the oxygen transport and diffusion in this candidate
electrolyte material. The main purpose of this project was to investigate oxide ion diffusion
in La2Mo2O9 and also the factors which could affect oxygen transport properties.
Oxygen isotope exchange followed by Secondary Ion Mass Spectrometry (SIMS)
measurements were employed to obtain oxygen diffusion profiles. A correlation between
oxygen ion transport and the electrochemical properties such as ionic conductivity was
built upon the Nernst Einstein equation relating the diffusivity to electrical conductivity.
In-situ neutron diffraction and AC impedance measurements were designed and conducted
to investigate the correlation between crystal structure and oxygen transport in the bulk
materials. Other techniques, such as synthesis, microstructure studies, and thermal analysis
were also adopted to study the electrochemical properties of La2Mo2O9.
The results of the study on the effects of microstructure on oxygen diffusion in
La2Mo2O9 revealed that the grain boundary component played a significant role in
electrochemical performance, although the grain size seemed to have little influence on
oxygen transport. The oxygen isotope exchange in 18O2 was successfully carried out by
introducing a silver coating on the sample surface, which solved the main difficulty in
applying oxygen isotope exchange on pure ionic conductors. The ionic conductivity
obtained from the diffusion coefficients was consistent with the result from AC impedance spectroscopy. The number of mobile oxygen ions was estimated to be 5 per unit cell. There
was a difference of oxygen self diffusion coefficient when the isotope exchange was
conducted in 18O2 and H2
18O. The activation energy of oxygen diffusion in humidified
atmosphere was higher than that measured in dry atmosphere. It indicated that the
humidified atmosphere had affected oxygen transport in the material. The studies on
hydroxyl incorporation and transport explained the decreased oxygen diffusion coefficients
in wet atmosphere and also suggested proton conductivity in La2Mo2O9, which leads to
further investigation on applications of La2Mo2O9 as a proton conductor. In-situ neutron
diffraction and AC impedance measurement revealed a close relationship between crystal
structure and ionic conductivity. The successful application of this technique provides a
new method to simultaneously investigate crystal structure and electrical properties in
electro-ceramics in the future.