Functional Materials for Energy

Topotactic valence state control in epitaxial multivalent oxides

Schematic diagram of the brownmillerite SrCoO2.5 structure with the alternating stack of octahedral and tetrahedral sublayers.

Valence state control is demonstrated in a multivalent transition metal oxide (TMO) by topotactic transformation, i.e. the change of crystal structure via loss or gain of constituent elements. As the dynamic change of the valence state in TMOs plays an essential role in the performance of many energy materials and devices, including solid oxide fuel cells, rechargeable batteries, gas sensors, and gas separation membranes, the topotactic valence state control via oxygen concentration may open new possibilities to fine tune key electrochemical processes.

A small change in the oxygen content results in the variation of valence state of the transition metal in TMOs, drastically modifying the global materials functionalities. However, a systematic study on this topotactic modification of the composition and associated physical properties has been hindered by the difficulty in synthesizing phase pure, single crystalline materials. In this study of strontium cobaltites, we have identified a very narrow growth window in which high quality brownmillerite SrCoO2.5 epitaxial thin films can be grown by pulsed laser epitaxy without any impurity phases. As the brownmillerite phase offers an interesting structurally open framework of one-dimensionally ordered oxygen vacancy channels (OVCs), this unique structure enables the material to gradually adopt oxygen and fill the OVCs. The topotactic phase transformation subsequently induces a structural phase transition with systematic and significant changes in electronic, magnetic, and thermoelectric properties. Moreover, since the open framework of the multivalent oxide offers a surface state that cannot be found in typical perovskite oxides, this work may significantly impact the development of high performance oxide-based sensors and cathodes.

H. Jeen, W. S. Choi, J. W. Freeland, H. Ohta, C. U. Jung, and H. N. Lee, “Topotactic phase transformation of the brownmillerite SrCoO2.5 to the perovskite SrCoO3-δ,” Adv. Mater. 25, 3651 (2013).

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