Hexagonal Rare Earth-Iron Mixed Oxides (REFeO₃): Crystal Structure, Synthesis, and Catalytic Properties

The rare earth-iron mixed oxide (REFeO₃) is an attractive material in fields such as electronic, magnetic, and catalytic research. Generally, orthorhombic REFeO₃ (o-REFeO₃) with a perovskite structure is better known than hexagonal REFeO₃ (h-REFeO₃), because o-REFeO₃ is thermodynamically stable for all RE elements. However, h-REFeO₃ 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-REFeO₃ 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-REFeO₃ and o-REFeO₃ with comparative ease. Although the electronic and magnetic properties of h-REFeO₃ 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 REFeO₃ and the catalytic performance of h-REFeO₃-based materials.