Crystal Structure of β-Fe₂O₃ and Topotactic Phase Transformation to α-Fe₂O₃

Abstract

β-Fe 2 O 3 is the scarce polymorph of Fe 2 O 3 phases and is transformed easily into α-Fe 2 O 3 at high temperature. However, its crystal structure and the transformation mechanism to α-Fe 2 O 3 are still unclear because of the difficulty in obtaining monophasic β-Fe 2 O 3 crystals. We established a synthesis method of the monophasic β-Fe 2 O 3 . It was synthesized by a two-step reaction: heating a mixture of Na 2 SO 4 and Fe 2 (SO 4 ) 3 in air at 250 °C to form NaFe(SO 4 ) 2, and subsequent heating the resultant phase with NaCl in air at 500 °C. The crystal structure was refined to a bixbyite-type cubic structure ( Ia 3̅) with a = 9.4039(1) Å by the Rietveld method. Single crystalline β-Fe 2 O 3 particles of approximately 1 μm in size were topotactically transformed into single α-Fe 2 O 3 crystals. Electron diffraction analysis revealed the crystallographic orientation relationships between β-Fe 2 O 3 and α-Fe 2 O 3 to be [100] β //[0001] α, [010] β //[101̅0] α, and [001] β //[1̅21̅0] α .