Synthesis of metastable rare-earth-iron mixed oxide with the hexagonal crystal structure

Rare-earth-iron mixed oxides with the rare earth/iron ratio=1 have either orthorhombic (o-REFeO ₃) or hexagonal (h-REFeO ₃) structure. h-REFeO ₃ is a metastable phase and the synthesis of h-REFeO ₃ is usually difficult. In this work, the crystallization process of the precursors obtained by co-precipitation and Pechini methods was investigated in detail to synthesize h-REFeO ₃. It was found that the crystallization from amorphous to hexagonal phase and the phase transition from hexagonal to orthorhombic phase occurred at a similar temperature range for rare earth elements with small ionic radii (Er-Lu, Y). For both co-precipitation and Pechini methods, single-phase h-REFeO ₃ was obtained by shortening the heating time during calcination process. The hexagonal-to-orthorhombic phase transition took place by a nucleation growth mechanism and vermicular morphology of the thus-formed orthorhombic phase was observed. The hexagonal YbFeO ₃ had higher catalytic activity for C ₃H ₈ combustion than orthorhombic YbFeO ₃.