Cation arrangement in iron–zinc–chromium spinel oxides

The distribution of ferric and ferrous ions within the quaternary Fe²⁺–Fe³⁺–Zn–Cr spinel solid solution series is documented using Mössbauer spectroscopy and X-ray diffraction. Along the binary joins FeCr₂O₄–Fe₃O₄, ZnCr₂O₄–Fe₃O₄ and ZnFe₂O₄–Fe₃O₄ the cation distribution changes from normal to inverse. From the different trends of iron distribution versus composition observed along these three joins and at other points within the Fe–Zn–Cr solid solution series, it was found that octahedral site ferric/ferrous charge hopping may be the most important phenomenon stabilizing the inverse ion distribution. For spinels whose compositions make charge hopping impossible or statistically unlikely, cation distribution is governed by the intrinsic spinel site preferences (tetrahedral site: Zn > Fe²⁺ Fe³⁺ >> Cr, octahedral site: Cr > Fe³⁺ > Fe²⁺ > Zn), which result from the size and bonding characteristics of these atoms.

Near the ZnFe₂O₄–Fe₃O₄ edge of this spinel quadrilateral, the normal to inverse transition proceeds essentially to the maximum extent allowed by the Zn content, because the octahedral Fe²⁺ produced by inversion can share electrons with the surrounding octahedral Fe³⁺ ions. Inversion along the FeCr₂O₄–Fe₃O₄ edge of the quadrilateral, however, is inhibited until there is a sufficient octahedral Fe³⁺ population to make electron sharing (with inversion produced octahedral Fe²⁺) statistically likely. A rapid change of site preference of iron ions occurs in the central range of this composition subspace, offering a physical reason for the miscibility gap found along the chromite–magnetite join.