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 iden tified as hematite (R(3) over bar c) but characterized by other selection r ules for IR and Raman active vibrations. Protohematite does not possess mec hanical stability. The protohematite-hematite phase transition is initiated by heating above 900 degreesC, or by pressure application, or by mechanoch emical 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 environ ment of some iron cations.