A Novel Strontium-Iron Oxide

A kinetically controlled synthesis has yielded a novel strontium-iron oxide with an unusual structure. The study, reported in Nature (2007, 450, 1062), may lead to new types of materials with useful properties, such as high ion mobility, which is essential to solid-oxide fuel cells and other applications.

The new compound, SrFeO₂, exhibits a so-called infinite-layer structure consisting of sheets of FeO₄ squares interleaved with strontium ions. That type of structure is adopted by copper oxide superconductors but has not been observed in iron oxides until now.

Mixed (or complex) metal oxides are typically prepared via solid-state methods at temperatures above 1,000°C. Under those conditions, reagents have sufficient energy to surmount high reaction barriers, and as such, the procedures generally afford the most thermodynamically stable products.

In contrast, solution-phase reactions, which tend to be unsuited to making complex metal oxides, generally call for much lower temperatures. With less energy available to drive chemical transformations, those reactions typically yield the product favored by kinetics, meaning the one that forms fastest. A key distinction between the two types of reactions is that in kinetically controlled processes, the product often bears a structural relationship to the starting material.

To prepare the new compound, Hiroshi Kageyama and Werner Paulus, chemistry professors at Kyoto University and the University of Rennes, respectively, and coworkers capitalized on a new low-temperature synthesis that sidesteps thermodynamic limitations of conventional solid-state reactions by using CaH₂ as a reducing agent.

Specifically, by treating an oxygen-deficient form of SrFeO₃ with CaH₂ at just 280°C, the team removed some of the oxide (O^2???) ions and formed an intermediate product, Sr₂Fe₂O₅. Then they repeated the procedure with the intermediate, stripping away more oxide ions and converting that compound to the previously unknown SrFeO₂. The analyses reveal that the strontium and iron ions retain their positions throughout the transformations, a result of the low-temperature conditions. Only the oxide ions rearrange.

In an accompanying commentary in Nature, the chemists who developed the hydride method, Michael A. Hayward of Oxford University and Matthew J. Rosseinsky of the University of Liverpool, note that the oxide ion mobility is a "crucial observation" that suggests a host of synthetic possibilities. "This achievement opens the door to synthesis of many other complex metal oxides with potentially useful properties," they write.