Shivani Srivastava1,Blas Uberuaga2,Mark Asta1,3
University of California, Berkeley1,Los Alamos National Laboratory2,Lawrence Berkeley National Laboratory3
Shivani Srivastava1,Blas Uberuaga2,Mark Asta1,3
University of California, Berkeley1,Los Alamos National Laboratory2,Lawrence Berkeley National Laboratory3
Magnetite (Fe<sub>3</sub>O<sub>4</sub>) is found as a constituent phase in oxide scales formed on iron under a variety of corrosion conditions. When exposed to an oxidizing corrosion environment, the point defects and their transport play an important role in determining the rates of corrosion.<br/>The work presented here employs first-principles calculations using PBE+U functional to investigate the role of local ordering of Fe<sup>2+</sup>/Fe<sup>3+</sup> cations in magnetite on the point defect properties occurring under equilibrium conditions. The bulk structure of magnetite, featuring coupled charge, spin and orbital order, undergoes a bulk order-disorder phase transition at around 120 K. Even above this transition, there is evidence of strong local order, consistent with results from calculations presented here. We study the effect of this local order on the energetics underlying defect formation and migration. Owing to the strong coupled charge, spin and orbital order, magnetite(Fe<sub>3</sub>O<sub>4</sub>) exhibits a strong dependence of defect properties on local charge order and underlying magnetic order.<br/>We focus on sampling multiple local environments and their associated effects on <br/>the underlying defect properties. Our results show that different local environments, generated due to ordering of Fe<sup>2+</sup>/Fe<sup>3+</sup> cations, stabilize different charge states of point defects such as oxygen and iron vacancies, depending on charge redistribution around the defect. We also find that iron-oxygen di-vacancies are stable in this system and have high binding energy under reducing conditions. The relatively high stabilization of defects in certain local chemical environments can lead to formation of percolation networks modifying the ionic transport in these systems. As shown in previous studies<sup>[1]</sup>, the interplay of defect formation and energetics of disordering plays an important role in determining radiation tolerance of complex oxides.The implications of our results for the effects of irradiation on magnetite formation during oxidation/corrosion of iron will be discussed.<br/><br/>[1] Kreller, C. R.; Uberuaga, B. P. The Role of Cation Ordering and Disordering on Mass Transport in Complex Oxides. Current Opinion in Solid State and Materials Science 2021, 25 (2), 100899. https://doi.org/10.1016/j.cossms.2021.100899.