The symmetry of ordered cubic γ-Fe2O3 investigated by TEM

Well-crystallized particles of cubic and tetragonal gamma-Fe2O3 embedded in a Pd matrix were produced besides other oxides by internal oxidation of a Pd-Fe alloy in air. Particles of tetragonal gamma-Fe2O3 consist of orientation domains with the c axes normal to each other. Particles of the ordered cubic gamma-Fe2O3 appear single crystalline in bright field and in dark field images with reflections of the basic spinel structure. In dark field images enantiomorphous domains were observed using reflections of the ordered phase. From the analysis of electron diffraction patterns in the principal zone axes the description of ordered cubic gamma-Fe2O3 in the enantiomorphous space groups P4(1)32/P4(3)32 follows without further presumptions. In the sequence from space group Fd (3) over barm of disordered cubic gamma-Fe2O3 via P4(1)32/P4(3)32 of the ordered cubic phase to the pair P4(1)2(1)2/P4(3)2(1)2 of tetragonal gamma-Fe2O3 a continuous group-subgroup relation can be derived. This relation shows that ordered cubic gamma-Fe2O3 is an intermediate phase upon ordering of vacant octahedral sites towards tetragonal gamma-Fe2O3.