Hexagonal Rare Earth-Iron Mixed Oxides (REFeO(3)): Crystal Structure, Synthesis, and Catalytic Properties

The rare earth-iron mixed oxide (REFeO(3)) is an attractive material in fields such as electronic, magnetic, and catalytic research. Generally, orthorhombic REFeO(3) (o-REFeO(3)) with a perovskite structure is better known than hexagonal REFeO(3) (h-REFeO(3)), because o-REFeO(3) is thermodynamically stable for all RE elements. However, h-REFeO(3) has a very interesting crystal structure in which a RE and Fe layer are alternately stacked along the c-axis in the unit cell; nevertheless, synthesis of the h-REFeO(3) belonging to metastable phase can be problematic. Fortunately, solution-based synthetic methods like solvothermal or coprecipitation synthesis have recently enabled the selective synthesis of h-REFeO(3) and o-REFeO(3) with comparative ease. Although the electronic and magnetic properties of h-REFeO(3) 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(3) and the catalytic performance of h-REFeO(3)-based materials.