Mass transport and electrochemical properties of La2Mo2O9 as a fast ionic conductor

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