Short range order in disordered spinels and the impact on cation vacancy transport

Spinels are important complex oxides for use in radiation damage environments and result from the corrosion of steel. It is known that, in these environments, normal spinels exist with some concentration of antisite cation pairs known as inversion. In this work we show that even in highly disordered states characterized by high levels of inversion, spinels still show some short range order (SRO) that manifests itself in antisite chains. The propensity to form these antisite chains is confirmed through Monte Carlo simulations which find that the length of chains which can form depends on the spinel chemistry. We also consider the effect of antisite chains on the diffusivity of cation vacancies and find that the effect strongly depends on the spinel chemistry. Two examples illustrate the range of chemistry dependance on cation vacancy diffusivity, chains in FeCr2O4 significantly increase vacancy transport but in MgAl2O4, they have the inverse effect of drastically reducing vacancy mobility. The explanation of these dramatically different effects results from the assessment of the thermodynamic stability of the antisite chains and contrasting attractive/repulsive interactions of vacancies with the chains.