Effect of lattice defects on proton conduction in Ba-based perovskite oxides

Abstract

The characteristics of the materials depend on the inherent characteristics of the materials itself. Therefore, it has been one of the main research directions in materials science to select materials showing various physical and chemical properties and control the microstructure of these materials. In addition, as devices become increasingly complex and miniaturized, various studies are underway to control the chemical and physical properties of the material to a more fundamental level. Because understanding and subsequent control of the formation and distribution of atomic-scale defects became central issues in many crystalline oxides, as the defects have a critical impact on the overall mass and charge transport and even the phase transformation. Therefore, chemical reaction control of physical structure at the atomic level can be regarded as a key field of basic research for development of devices with new conceptual performance. By using defect control and atomic-scale direct observation method in this thesis, we studied the change of physical and chemical properties perovskite oxide materials which used as solid electrolyte for protonic ceramic fuel cell. In addition, pre-melting phenomena in the Ruddlesden-Popper faults at internal perovskite oxide were observed directly by atomic-scale observation. The first part of the thesis is the result of improving the bulk proton conduction in perovskite structure. In particular, we have focused on the influence of acceptoroxygen-vacancy clusters on proton conduction by using a combination of density functional theory (DFT) calculations and impedance spectroscopy at elevated temperatures. Among many proton-conducting oxides, acceptor-doped $BaZrO_3$ is one of the well-known and extensively studied candidates because of its excellent chemical stability and high proton conductivity. While binding interaction between acceptor dopants and protons, which is referred to as proton trapping, has been intensively studied in $BaZrO_3$, the effect of defect clusters consisting of an acceptor and an oxygen vacancy on proton conduction has remained elusive. We revealed that such trapping behavior of protons can be significantly alleviated when the acceptors are clustered with oxygen vacancies using DFT calculations. To suppress the high-temperature entropy and thus make oxygen vacancies cluster with acceptors, post-annealing was adopted at substantially lower temperatures than those employed for sintering processes. The acceptorvacancy clustering was verified to be remarkably efficient against proton trapping by experimental measurements based on impedance spectroscopy along with atomic-scale direct visualization. Therefore, this study demonstrates that the strong electrostatic trapping of protons to neighboring acceptors in $BaZrO_3$ can be remarkably overcome via control of oxygen-vacancy distribution in the lattice, implying that such defect clustering is an effective approach to enhancing proton conduction in other acceptor-doped perovskite oxides. Along with the bulk in proton conducting perovskite oxide, grain boundaries are also generally accepted as rate-limiting obstacles to rapid ionic diffusion, often resulting in overall sluggish transport. Consequently, based on a precise understanding of the structural and compositional features at grain boundaries, systematic control of the polycrystalline microstructure is a key factor to achieve better ionic conduction performance. In the second part of this thesis, we clarify that a nanometer-thick amorphous phase at most grain boundaries in proton-conducting $BaCeO_3$ polycrystals is responsible for substantial retardation of proton migration and moreover is very reactive with water and carbon dioxide gas. By a combination of atomic-scale chemical analysis and physical imaging, we demonstrate that highly densified $BaCeO_3$ polycrystals free of a grain-boundary amorphous phase can be easily fabricated by a conventional ceramic process and show sufficiently high proton conductivity together with significantly improved chemical stability. These findings emphasize the value of direct identification of intergranular phases and subsequent manipulation of their distribution in ion-conducting oxide polycrystals. Even though the $BaCeO_3$ perovskite system has been widely well known and studied as a proton conducting solid electrolyte exhibiting high proton conductivities, precise analysis in the atomic-scale are still insufficient for understanding the detailed information about defect formation, proton conduction, and even phase transition mechanism. In the last part of this thesis, the pre-melting phenomenon at the ruddlesden popper faults is observed through atomic-scale direct observation method.