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

β-Fe2O3 is the scarce polymorph of Fe2O3 phases and is transformed easily into α-Fe2O3 at high temperature. However, its crystal structure and the transformation mechanism to α-Fe2O3 are still unclear because of the difficulty in obtaining monophasic β-Fe2O3 crystals. We established a synthesis method of the monophasic β-Fe2O3. It was synthesized by a two-step reaction: heating a mixture of Na2SO4 and Fe2(SO4)3 in air at 250 °C to form NaFe(SO4)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 (Ia3̅) with a = 9.4039(1) Å by the Rietveld method. Single crystalline β-Fe2O3 particles of approximately 1 μm in size were topotactically transformed into single α-Fe2O3 crystals. Electron diffraction analysis revealed the crystallographic orientation relationships between β-Fe2O3 and α-Fe2O3 to be [100]β//[0001]α, [010]β//[101̅0]α, and [001]β//[1̅21̅0]α.