Structure of the metastable modification of iron(III) oxide

A new metastable modification of iron(III) oxide — protohematite — has_been studied. According to the X-ray diffractogram, protohematite (R3c) is identified as hematite (R3c) but characterized by other selection rules for IR and Raman active vibrations. Protohematite does not possess mechanical stability. The protohematite-hematite phase transition is initiated by heating above 900°C, or by pressure application, or by mechanochemical activation. The higher catalytic activity of protohematite compared to hematite may be explained by its structural features, namely, by lowered symmetry of the oxygen sublattice and by the effective tetrahedral environment of some iron cations. Translated fromZhurnal Strukturnoi Khimii, Vol. 41, No. 3, pp. 489-497, May—June, 2000.