Hexagonal Rare Earth-Iron Mixed Oxides (REFeO): Crystal Structure, Synthesis, and Catalytic Properties.

The rare earth-iron mixed oxide (REFeO) is an attractive material in fields such as electronic, magnetic, and catalytic research. Generally, orthorhombic REFeO (-REFeO) with a perovskite structure is better known than hexagonal REFeO (-REFeO), because REFeO is thermodynamically stable for all RE elements. However, -REFeO has a very interesting crystal structure in which a RE and Fe layer are alternately stacked along the -axis in the unit cell; nevertheless, synthesis of the -REFeO belonging to metastable phase can be problematic. Fortunately, solution-based synthetic methods like solvothermal or coprecipitation synthesis have recently enabled the selective synthesis of -REFeO and -REFeO with comparative ease. Although the electronic and magnetic properties of -REFeO have typically been evaluated, recent research has also revealed excellent catalytic properties that enable environmental cleanup reactions such as hydrocarbon or CO oxidation. This mini-review introduces a synthetic method for controlling the crystal structure between orthorhombic and hexagonal REFeO and the catalytic performance of -REFeO-based materials.