(262a) A Fundamental Understanding of Cation Transport in Mixed Metal Ferrite Spinels

Magnetite (Fe₃O₄) and its alloys are used in wide variety of materials and chemical engineering applications and reactions. In several of these applications the performance of the ferrite spinels are controlled by the rates of cationic diffusion through the lattice. In this work we develop a fundamental understanding of the diffusion process using ab initio calculations based on density functional theory. Specifically, we investigate the diffusion of Fe, Co and Ni cations through three difference ferrite spinels (Fe₃O₄, CoFe₂O₄, and NiFe₂O₄). The cations diffuse via a vacancy mediated octahedral-octahedral hopping mechanism where the cations hop through a trigonal planer transition state to a tetrahedral meta-stable state. The individual hopping rate depends on both the diffusing cation and the material. The cations have relative activation energies of Co â??< Fe < Ni. This ordering is controlled by the change in cation coordination along the reaction path which leads to changes in electronic ordering of the diffusant cationsâ?? t_2g and e_g orbitals due to crystal field effects. Diffusing cations with higher occupancy of the t_2g and e_g orbitals have higher activation energies because more electrons must be promoted to higher energy orbitals during hopping events. An inverse relationship exists between the lattice constant of a material and the hopping activation energy, leading to relative activation energies for the various spinels of Fe₃O₄ < CoFe₂O₄ < NiFe₂O₄. This behavior is attributed to increased steric interactions between the diffusing cation and the O anions in the materials with smaller lattices.