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.