Study On The Oxygen-Ion Diffusion And Conduction Mechanism Of La₂Mo₂O₉-Based Novel Oxide-Ion Conductors

Oxide-ion conductors have been received considerable attention owing to their potential applications in solid oxide fuel cells, oxygen sensors, oxygen pumping devices in the recent years, and have become one of the most important field and focus of solid electrolyte materials. The aim of this thesis is to research the microscopic relaxation mechanism of oxygen-ion diffusion and conduction properties of the La2Mo2O9-based novel oxide-ion conductors. The main content and innovation are described as follows: The dissertation consists of six chapters. Chapter one is an introduction. At first we present an overview of the research progress and potential application of oxide-ion conductors. Based on the LPS (Lone-pair substitution) theory, we especially introduce the crystal structure, formation mechanism of intrinsic oxygen vacancy and conduction property of the novel oxide-ion conductors La2Mo2O9. And then, the internal friction and dielectric relaxation techniques are also simply descried as the main measurement methods in our research work. In the end of this chapter, we present our main goals. In chapter two, the microscopic transport mechanism of oxygen ions in oxide-ion conductor La2Mo2O9 has been investigated by means of internal friction and dielectric relaxation methods, and its dynamical relaxation parameters are also deduced. In the internal friction measurement, two internal friction (IF) peaks are observed around 380 K and 833 K at a measurement frequency of 1 Hz. The lower-temperature peak is of relaxation type and actually composed of two sub-peaks (termed as P1 peak and P2 peak, respectively) that originate from the two different kinds of diffusion paths of oxygen ions in La2Mo2O9. As for the higher-temperature peak, it is associated with a first-order phase transition, which corresponds to an order-disorder transition of oxygen ion distribution. In the dielectric experiment, an apparent relaxation peak (termed as Pd peak) associated with the oxygen ion diffusion is also observed. The activation energies and the relaxation time at infinite temperature are determined as (0.9 eV, 3×10-16 s), (1.1 eV, 2×10-16 s) and (0.99 eV, 5×10-14 s) for the P1 peak, P2 peak and Pd peak, respectively. The measurement results of the relaxation parameters further conformed the relaxation nature of oxygen ion diffusion. Based on the crystalline structure of La2Mo2O9, an atomistic mechanism of oxygen ion diffusion via vacancies is suggested. In chapter three, we have mainly studied the effects of Ca doping on the oxygen ion diffusion and phase transition in oxide-ion conductors La2Mo2O9, and found that the relaxation strength of the IF peak dramatically decreased at first but then slowly increased with increasing of Ca-doping concentration, corresponding to the changes of lattice constant and activation energy. The results of conductivity measurement show that the ionic conductivity at lower-temperature can be improved in some degree when the Ca-doping content reaches to 30%. Chapter four mainly concerns the K-doping effects on oxygen ion diffusion in oxide-ion conductors La2Mo2O9. It is found that the only one dielectric relaxation peak, observed in pure La2Mo2O9, was spilt into two peaks, and relaxation strength also greatly decreased after K doping. The main reasons of these phenomena can be attributed to the decrease of unit-cell free volume in lattice after the partial substitution of La3+ with K+. However, the blocking effects of K+ ions are not the same on different diffusion process of oxygen ions, the mainly affected jumping path is O(1)?O(3) . As for the electric property of oxide-ion conductor La2-xKxMo2O9-δ, the ionic conductivity at lower-temperature can be improved by one order of magnitude in the case of 5% K doping, in comparison with that of pure La2Mo2O9. This improvement of conductivity is very important to enhance the potential application of such kind of materials. In chapter five, we focus our attention on the blocking effects of the oxygen iondiffusion by lone-pair electrons, which were introduced by the partial substitution of La3+ with Bi3+. In the dielectric spectra, we observed two relaxation peaks associated with oxygen ion diffusion in oxide-ion conductors La2-xBixMo2O9. With increasing the Bi-doping concentration, the activation energies of the two relaxation peaks are found to increase. This can be reasonably explained by the blocking effects of the lone-pair electron in Bi3+ ions. From the different effects of Bi-doping on the two dielectric peaks, the diffusion paths of oxygen ions corresponding to each peak are further confirmed. Significantly, it is revealed that the Bi doping could also effectively enhance the ionic conductivity of La2Mo2O9 at lower-temperature. Chapter six present a summary of this thesis.