Kinetics of Cation Redisribution in Ferrospinels

In situ electrical resistivity measurements were employed to follow the kinetics of cation redistribution in ferrospinels, xFe₃O₄–(1 –x)MeFe₂O₄ (Me = Co, Mg, Mn, or Ni) or xFe₃O₄–FeMe₂O₄ (Me = Al) with x≥ 0.2. Relaxation temperatures (at 20°C/min heating rate) were established and kinetic parameters—activation energies and time constants—were determined for each cation. These parameters were insensitive to grain size, cation ratio, and oxygen nonstoichiometry. The mechanism was shown to be a local “homogeneous” point defect reaction in contrast to the “heterogeneous” nucleation and growth mechanism reported to take place in ferrispinels lacking Fe²⁺. Electron hopping between Fe²⁺ and Fe³⁺ in ferrospinels apparently screens the excess ionic charge associated with the diffusing species and enables homogeneous point defect reactions with lower attendant activation energies to take place.