Abstract:
Oxide-ion conductors are of high interest in electrochemical devices such as solid-oxide fuel cells, oxygen sensors, and separation membranes. In this paper, high oxide-ion conductivity and associated ion conduction mechanism in perovskite-type oxides Na sub(0.5) Bi sub(0.5-x) Ti sub(1-y) Mg sub(y) O sub(3-1.5x-y) (for x = 0.0 and y = 0.0, x = 0.01 and y = 0.02, x = 0.01 and y = 0.04) are investigated systematically. Na sub(0.5) Bi sub(0.5) TiO sub(3) ceramic is a poor conductor, whereas Na sub(0.5)Bi sub(0.49) Ti sub(0.98) Mg sub(0.02) O sub(2.965) and Na sub(0.5) Bi sub(0.49) Ti sub(0.96) Mg sub(0.04) O sub(2.945) ceramics are excellent oxide-ion conductors at 500 degrees C. While the Rietveld refinements of powder X-ray diffraction data using the monoclinic Cc space group and the rhombohedral R3c space group showed reasonably similar quality of fits, extended X-ray absorption fine structure (EXAFS) data could be fitted only with the monoclinic Cc structure at room temperature for all three ceramics. Extensive EXAFS investigations have also been used to probe the local environments of Bi and Ti atoms directly and reveal the ordering of Bi sup(3+)/Na sup(+), displacements of the cations, oxygen-vacancy generation, and their migration pathways. Our EXAFS results demonstrate Bi- and Na-rich planes formation due to short-range ordering of Bi sup(3+)/Na sup(+) in the perovskite units. Oxygen vacancies were found to be located in the Bi-rich planes. sup(23) Na magic-angle spinning NMR experiments indicate that the local environments of Na atoms are disordered. The present work also provides an insight into the dramatically improved conducting behavior of Na sub(0.5) Bi sub(0.49) Ti sub(0.98) Mg sub(0.02) O sub(2.965) and Na sub(0.5) Bi sub(0.49) Ti sub(0.96) Mg sub(0.04) O sub(2.945) ceramics in terms of the local, long-range, and microstructure, which can be exploited to develop design principles for the syntheses of related oxides with even improved properties.
