Spin-state transition of iron in (Ba0.5Sr0.5)(Fe0.8Zn0.2)O3-δ perovskite

The redox behavior of iron during heating of a high-performance perovskite for ceramic oxygen separation membranes was studied by combined electron energy-loss (EELS, esp. ELNES) and Mossbauer spectroscopical in situ methods. At room temperature, the iron in (Ba0.5Sr0.5) (Fe0.8Zn0.2)O3-delta (BSFZ) is in a mixed valence state of 75% Fe4+ in the high-spin state and 25% Fe3+ predominantly in the low-spin state. When heated to 900 degrees C, a slight reduction of iron is observed that increases the quantity of Fe3+ species. However, the dominant occurrence is a gradual transition in the spin-state of trivalent iron from a mixed low-spin/high-spin to a pure high-spin configuration. in addition, a remarkable amount of hybridization is found in the Fe-O bonds that are highly polar rather than purely ionic. The coupled valence/spin-state transition correlates with anomalies in thermogravimetry and thermal expansion behavior observed by X-ray diffraction and dilatometry, respectively. Since the effective cationic radii depend not only on the valence but also on the spin-state, both have to be considered when estimating under which conditions a cubic perovskite will tolerate specific cations. It is concluded that an excellent phase stability of perovskite-based membrane materials demands a tailoring, which enables pure high-spin states under operational conditions, even if mixed valence states are present. The low spin-state transition temperature of BSFZ provides that all iron species are in a pure high-spin configuration already above ca. 500 degrees C making this ceramic highly attractive for intermediate temperature applications (500-800 degrees C). (C) 2009 Elsevier Inc. All rights reserved.