2017 May 17 - June 29
2017 October 11 - December 21
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Among the many exciting potential applications of complex oxide materials are their use in solid oxide fuel cells, oxygen sensors, and other applications requiring oxygen anion diffusion. Some of these materials can form distinct, oxygen-deficient, ordered phases with high ionic conductivity, but fabricating such phases poses a challenge. Now, researchers at Cornell University and CHESS have discovered a new way to fabricate such materials using standard thin film deposition techniques. The method involves depositing "normal," oxygen rich material first, followed by a oxygen poor layer which, due to greater oxygen affinity, extracts oxygen from the original, buried film. As oxygen is extracted from the buried layer, it undergoes a phase change to a so-called Brownmillerite phase, whose full unit cell is approximately four times that of the original crystal.
a) Scanning transmission electron microscopy image of a complex oxide film. The oxygen vacancies have ordered into a brownmillerite type structure. b) The idealized brownmillerite unit cell, ABO2.5, with the corresponding perovskite unit cell shown in green. A site atoms are green spheres, B site blue spheres, and oxygen atoms are shown as red spheres.
The serendipitous discovery was made largely thanks to the fact that the growth process was being monitored in situ, using x-rays generated at CHESS. Because the remarkable phenomena occurs in a buried layer, it is invisible to comparable techniques based on electron diffraction, such as RHEED. Moreover, the ordered phase disappears after its formation unless particular procedures are followed after growth is complete. Thus, it is possible that such phases have occurred in many other laboratories, but gone un-noticed. The results appeared online in the journal, Advanced Materials in January, 2011:
Submitted by: Arthur Woll, CHESS, Cornell University