An approach to understanding the formation mechanism of NiFe₂O₄ inverse spinel

The role of diffusion in the formation of the inverse spinel phase, as opposed to the normal phase which is never observed, is still not well understood. This is in part due to the difficulty or impossibility to do in situ experiments to observe how the inverse spinel phase forms, owing to the small reaction/transition times and equipment sensitivity. Here, we show that diffusion of point defects such as cation interstitials is responsible for the transition from normal to inverse spinel, which explains why a normal phase is never observed. These mechanisms are studied computationally using the kinetic activation -relaxation technique (k -ART), an off -lattice kinetic Monte Carlo algorithm. We find, in particular, how Fe and Ni kinetics both support the Ni interstitial diffusion with low activation barriers. The inverse spinel structure is analyzed by Mossbauer and XRD ex situ experimental results which corroborate inverse spinel formation.